With gateway.rocket.chat:
Dec 19 21:13:12 rocketchat RocketChat: Push: Send message "@aTestUser" via query { userId: 'aTestUserID' }
Dec 19 21:13:12 rocketchat RocketChat: send to token { gcm: 'DeletedForPrivacy' }
Dec 19 21:13:12 rocketchat RocketChat: sendGCM [ 'DeletedForPrivacy' ] { _id: 'wcHz4GX2CgJu7oCmw',
Dec 19 21:13:12 rocketchat RocketChat: createdAt: 2017-12-19T20:13:12.846Z,
Dec 19 21:13:12 rocketchat RocketChat: createdBy: '<SERVER>',
Dec 19 21:13:12 rocketchat RocketChat: from: 'push',
Dec 19 21:13:12 rocketchat RocketChat: title: '@aTestUser',
Dec 19 21:13:12 rocketchat RocketChat: text: 'This is a push test message',
Dec 19 21:13:12 rocketchat RocketChat: sound: 'default',
Dec 19 21:13:12 rocketchat RocketChat: apn: { text: '@aTestUser:\nThis is a push test message' },
Dec 19 21:13:12 rocketchat RocketChat: query: { userId: 'aTestUserID' },
Dec 19 21:13:12 rocketchat RocketChat: sent: false,
Dec 19 21:13:12 rocketchat RocketChat: sending: 0 }
Dec 19 21:13:12 rocketchat RocketChat: Create GCM Sender using "MyAPIKeyCreatedFromTheDocumentation"
Dec 19 21:13:12 rocketchat RocketChat: A:Send message to: DeletedForPrivacy
Dec 19 21:13:12 rocketchat RocketChat: Push: Sent message "@aTestUser" to 0 ios apps 1 android apps
Dec 19 21:13:13 rocketchat RocketChat: ANDROID: Result of sender: {"multicast_id":7073480436697551000,"success":0,"failure":1,"canonical_ids":0,"results":[{"error":"MismatchSenderId"}]}

Without gateway.rocket.chat (after restarting the server):
Dec 19 21:21:22 rocketchat RocketChat: Push: Send message "@aTestUser" via query { userId: 'aTestUserID' }
Dec 19 21:21:22 rocketchat RocketChat: send to token { gcm: 'DeletedForPrivacy' }
Dec 19 21:21:22 rocketchat RocketChat: sendGCM [ 'DeletedForPrivacy' ] { _id: 'e7Zh4ZQ9HgNHcKfAy',
Dec 19 21:21:22 rocketchat RocketChat: createdAt: 2017-12-19T20:21:22.332Z,
Dec 19 21:21:22 rocketchat RocketChat: createdBy: '<SERVER>',
Dec 19 21:21:22 rocketchat RocketChat: from: 'push',
Dec 19 21:21:22 rocketchat RocketChat: title: '@aTestUser',
Dec 19 21:21:22 rocketchat RocketChat: text: 'This is a push test message',
Dec 19 21:21:22 rocketchat RocketChat: sound: 'default',
Dec 19 21:21:22 rocketchat RocketChat: apn: { text: '@aTestUser:\nThis is a push test message' },
Dec 19 21:21:22 rocketchat RocketChat: query: { userId: 'aTestUserID' },
Dec 19 21:21:22 rocketchat RocketChat: sent: false,
Dec 19 21:21:22 rocketchat RocketChat: sending: 0 }
Dec 19 21:21:22 rocketchat RocketChat: Create GCM Sender using "MyAPIKeyCreatedFromTheDocumentation"
Dec 19 21:21:22 rocketchat RocketChat: A:Send message to: DeletedForPrivacy
Dec 19 21:21:22 rocketchat RocketChat: Push: Sent message "@aTestUser" to 0 ios apps 1 android apps
Dec 19 21:21:22 rocketchat RocketChat: ANDROID: Result of sender: {"multicast_id":7262132309631144000,"success":0,"failure":1,"canonical_ids":0,"results":[{"error":"MismatchSenderId"}]}