Warning, OT: none of the apps you mention actually synchronise anything. Apart from Google+ Messaging, all of the apps you mention access a single (central) sou

Warning, OT: none of the apps you mention actually synchronise anything. Apart from Google+ Messaging, all of the apps you mention access a single (central) source of content. Google+ Messaging, on the other hand, creates content locally, rather than centrally. Syncing content implies sharing it between each device that needs it. That may or may not include a central server.