gem 'google-api-fcm'

require 'google/apis/messages'

# More Google Auth examples you can find in original gem
scope = Google::Apis::Messages::AUTH_MESSAGES
authorization = Google::Auth.get_application_default(scope)


service = Google::Apis::Messages::MessagesService.new(project_id: 'PROJECT-ID')
service.authorization = authorization

# Build notification message (with topic)
message = Google::Apis::Messages::Message.new(
  topic: 'news',
  title: 'Breaking news',
  body: 'Hell has frozen over'
)

# Build notification message (with token)
message = Google::Apis::Messages::Message.new(
  token: 'bk3RNwTe3H0:CI2k_HHwgIpoDKCIZvvDMExUdFQ3P1',
  title: 'Breaking news',
  body: 'Hell has frozen over'
)

# Finally send the message to FCM
service.notify(message)