GtkApp::TextBuffer

#...
def setup_ui
  @view.txtComment.buffer = GtkApp::TextBuffer.new
end

%w[Bold Italic Underline Strikethrough].each do |name|
  on :"btn#{name}", 'clicked' do |widget|
    @view.txtComment.buffer.format_selection(name.downcase.to_sym)
  end
end

%w[Redo Undo].each do |name|
  on :"btn#{name}", 'clicked' do |widget|
    @view.txtComment.buffer.send(name.to_sym)
  end
end
#...