diff --git a/changelog.d/372.feature b/changelog.d/372.feature
new file mode 100644
index 00000000..7cc2cd4d
--- /dev/null
+++ b/changelog.d/372.feature
@@ -0,0 +1 @@
+FCM v1: use async version of google-auth and add HTTP proxy support.
diff --git a/pyproject.toml b/pyproject.toml
index a4351ffd..2b6955e7 100644
--- a/pyproject.toml
+++ b/pyproject.toml
@@ -78,7 +78,7 @@ dependencies = [
     "attrs>=19.2.0",
     "cryptography>=2.6.1",
     "idna>=2.8",
-    "google-auth>=2.27.0",
+    "google-auth[aiohttp]>=2.27.0",
     "jaeger-client>=4.0.0",
     "matrix-common==1.3.0",
     "opentracing>=2.2.0",
@@ -104,6 +104,7 @@ dev = [
     "mypy-zope==1.0.1",
     "towncrier",
     "tox",
+    "google-auth-stubs==0.2.0",
     "types-opentracing>=2.4.2",
     "types-pyOpenSSL",
     "types-PyYAML",
diff --git a/sygnal/gcmpushkin.py b/sygnal/gcmpushkin.py
index 621c2fd7..010dd591 100644
--- a/sygnal/gcmpushkin.py
+++ b/sygnal/gcmpushkin.py
@@ -14,18 +14,24 @@
 # WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 # See the License for the specific language governing permissions and
 # limitations under the License.
+import asyncio
 import json
 import logging
+import os
 import time
 from enum import Enum
 from io import BytesIO
 from typing import TYPE_CHECKING, Any, AnyStr, Dict, List, Optional, Tuple
 
-import google.auth.transport.requests
-from google.oauth2 import service_account
+# We are using an unstable async google-auth API, but it's there since 3+ years
+# https://github.com/googleapis/google-auth-library-python/issues/613
+import aiohttp
+import google.auth.transport._aiohttp_requests
+from google.auth._default_async import load_credentials_from_file
+from google.oauth2._credentials_async import Credentials
 from opentracing import Span, logs, tags
 from prometheus_client import Counter, Gauge, Histogram
-from twisted.internet.defer import DeferredSemaphore
+from twisted.internet.defer import Deferred, DeferredSemaphore
 from twisted.web.client import FileBodyProducer, HTTPConnectionPool, readBody
 from twisted.web.http_headers import Headers
 from twisted.web.iweb import IResponse
@@ -180,10 +186,33 @@ def __init__(self, name: str, sygnal: "Sygnal", config: Dict[str, Any]) -> None:
                 "Must configure `project_id` when using FCM api v1",
             )
 
-        self.service_account_file = self.get_config("service_account_file", str)
-        if self.api_version is APIVersion.V1 and not self.service_account_file:
-            raise PushkinSetupException(
-                "Must configure `service_account_file` when using FCM api v1",
+        self.credentials: Optional[Credentials] = None
+
+        if self.api_version is APIVersion.V1:
+            self.service_account_file = self.get_config("service_account_file", str)
+            if not self.service_account_file:
+                raise PushkinSetupException(
+                    "Must configure `service_account_file` when using FCM api v1",
+                )
+            try:
+                self.credentials, _ = load_credentials_from_file(
+                    str(self.service_account_file),
+                    scopes=AUTH_SCOPES,
+                )
+            except google.auth.exceptions.DefaultCredentialsError as e:
+                raise PushkinSetupException(
+                    f"`service_account_file` must be valid: {str(e)}",
+                )
+
+            session = None
+            if proxy_url:
+                # `ClientSession` can't directly take the proxy URL, so we need to
+                # set the usual env var and use `trust_env=True`
+                os.environ["HTTPS_PROXY"] = proxy_url
+                session = aiohttp.ClientSession(trust_env=True, auto_decompress=False)
+
+            self.google_auth_request = google.auth.transport._aiohttp_requests.Request(
+                session=session
             )
 
         # Use the fcm_options config dictionary as a foundation for the body;
@@ -464,21 +493,26 @@ def _handle_v1_response(
                 f"Unknown GCM response code {response.code}"
             )
 
-    def _get_access_token(self) -> str:
-        """Retrieve a valid access token that can be used to authorize requests.
+    async def _get_auth_header(self) -> str:
+        """Retrieve the auth header that can be used to authorize requests.
 
-        :return: Access token.
+        :return: Needed content of the `Authorization` header
         """
-        # TODO: Should we use the environment variable approach instead?
-        # export GOOGLE_APPLICATION_CREDENTIALS=/path/to/key.json
-        # credentials, project = google.auth.default(scopes=AUTH_SCOPES)
-        credentials = service_account.Credentials.from_service_account_file(
-            str(self.service_account_file),
-            scopes=AUTH_SCOPES,
-        )
-        request = google.auth.transport.requests.Request()
-        credentials.refresh(request)
-        return credentials.token
+        if self.api_version is APIVersion.Legacy:
+            return "key=%s" % (self.api_key,)
+        else:
+            assert self.credentials is not None
+            await self._refresh_credentials()
+            return "Bearer %s" % self.credentials.token
+
+    async def _refresh_credentials(self) -> None:
+        assert self.credentials is not None
+        if not self.credentials.valid:
+            await Deferred.fromFuture(
+                asyncio.ensure_future(
+                    self.credentials.refresh(self.google_auth_request)
+                )
+            )
 
     async def _dispatch_notification_unlimited(
         self, n: Notification, device: Device, context: NotificationContext
@@ -532,10 +566,7 @@ async def _dispatch_notification_unlimited(
                 "Content-Type": ["application/json"],
             }
 
-            if self.api_version == APIVersion.Legacy:
-                headers["Authorization"] = ["key=%s" % (self.api_key,)]
-            elif self.api_version is APIVersion.V1:
-                headers["Authorization"] = ["Bearer %s" % (self._get_access_token(),)]
+            headers["Authorization"] = [await self._get_auth_header()]
 
             body = self.base_request_body.copy()
             body["data"] = data
diff --git a/tests/test_gcm.py b/tests/test_gcm.py
index a5454937..60c3d960 100644
--- a/tests/test_gcm.py
+++ b/tests/test_gcm.py
@@ -13,10 +13,11 @@
 # See the License for the specific language governing permissions and
 # limitations under the License.
 import json
+import tempfile
 from typing import TYPE_CHECKING, Any, AnyStr, Dict, List, Tuple
 from unittest.mock import MagicMock
 
-from sygnal.gcmpushkin import GcmPushkin
+from sygnal.gcmpushkin import APIVersion, GcmPushkin
 
 from tests import testutils
 from tests.testutils import DummyResponse
@@ -79,6 +80,21 @@
 }
 
 
+class TestCredentials:
+    def __init__(self) -> None:
+        self.valid = False
+
+    @property
+    def token(self) -> str:
+        if self.valid:
+            return "myaccesstoken"
+        else:
+            raise Exception()
+
+    async def refresh(self, request: Any) -> None:
+        self.valid = True
+
+
 class TestGcmPushkin(GcmPushkin):
     """
     A GCM pushkin with the ability to make HTTP requests removed and instead
@@ -92,6 +108,8 @@ def __init__(self, name: str, sygnal: "Sygnal", config: Dict[str, Any]):
         self.last_request_body: Dict[str, Any] = {}
         self.last_request_headers: Dict[AnyStr, List[AnyStr]] = {}  # type: ignore[valid-type]
         self.num_requests = 0
+        if self.api_version is APIVersion.V1:
+            self.credentials = TestCredentials()  # type: ignore[assignment]
 
     def preload_with_response(
         self, code: int, response_payload: Dict[str, Any]
@@ -110,8 +128,27 @@ async def _perform_http_request(  # type: ignore[override]
         self.num_requests += 1
         return self.preloaded_response, json.dumps(self.preloaded_response_payload)
 
-    def _get_access_token(self) -> str:
-        return "token"
+    async def _refresh_credentials(self) -> None:
+        assert self.credentials is not None
+        if not self.credentials.valid:
+            await self.credentials.refresh(self.google_auth_request)
+
+
+FAKE_SERVICE_ACCOUNT_FILE = b"""
+{
+  "type": "service_account",
+  "project_id": "project_id",
+  "private_key_id": "private_key_id",
+  "private_key": "-----BEGIN PRIVATE KEY-----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\\n7sGEnS/d8wEgydMiptJEf9s=\\n-----END PRIVATE KEY-----\\n",
+  "client_email": "firebase-adminsdk@project_id.iam.gserviceaccount.com",
+  "client_id": "client_id",
+  "auth_uri": "https://accounts.google.com/o/oauth2/auth",
+  "token_uri": "https://oauth2.googleapis.com/token",
+  "auth_provider_x509_cert_url": "https://www.googleapis.com/oauth2/v1/certs",
+  "client_x509_cert_url": "https://www.googleapis.com/robot/v1/metadata/x509/firebase-adminsdk%40project_id.iam.gserviceaccount.com",
+  "universe_domain": "googleapis.com"
+}
+"""
 
 
 class GcmTestCase(testutils.TestCase):
@@ -128,11 +165,14 @@ def config_setup(self, config: Dict[str, Any]) -> None:
             "api_key": "kii",
             "fcm_options": {"content_available": True, "mutable_content": True},
         }
+        self.service_account_file = tempfile.NamedTemporaryFile()
+        self.service_account_file.write(FAKE_SERVICE_ACCOUNT_FILE)
+        self.service_account_file.flush()
         config["apps"]["com.example.gcm.apiv1"] = {
             "type": "tests.test_gcm.TestGcmPushkin",
             "api_version": "v1",
             "project_id": "example_project",
-            "service_account_file": "/path/to/file.json",
+            "service_account_file": self.service_account_file.name,
             "fcm_options": {
                 "apns": {
                     "payload": {
@@ -146,6 +186,9 @@ def config_setup(self, config: Dict[str, Any]) -> None:
             },
         }
 
+    def tearDown(self) -> None:
+        self.service_account_file.close()
+
     def get_test_pushkin(self, name: str) -> TestGcmPushkin:
         pushkin = self.sygnal.pushkins[name]
         assert isinstance(pushkin, TestGcmPushkin)
@@ -260,6 +303,10 @@ def test_expected_api_v1(self) -> None:
         )
 
         self.assertEqual(resp, {"rejected": []})
+        assert notification_req[3] is not None
+        self.assertEqual(
+            notification_req[3].get("Authorization"), ["Bearer myaccesstoken"]
+        )
 
     def test_expected_with_default_payload(self) -> None:
         """