I don't think they do at all. Most of Google's core services are built on top of standards like SMTP, IMAP, CalDAV, Jabber, etc, so they can be consumed by any

I don't think they do at all. Most of Google's core services are built on top of standards like SMTP, IMAP, CalDAV, Jabber, etc, so they can be consumed by any device. Apple is the one writing most of the extra code here because they want to make sure Google's services run smoothly on their platform. While we joke about how crappy it is now, Apple's iTunes for Windows was a good example of playing nice on a competitors platform in an attempt to impress their customers and bring them to your side. It worked.