This happens to me with appointments I export from Outlook 2010 into Google that then syncs up with Calendar. NOT with events I enter manually into the iPhone, or enter in Calendar on any of my macs running Mavericks or if I enter events directly into google cal.
GMT only show up on the iPhone, nowhere else. I'm guessing maybe something is in the file I'm importing into google that gets passed on to iOS 8. I do wish I could turn it off, it is annoying and I don't want to have to re-enter appts. Sure wish Microsoft Outlook, Apple Calendar and Google Cal would play nice, but I guess that's no more likely than world peace.
Not sure I'd call this a bug with all the syncing and exporting I have going on, probably something relating to timezone support?? but would be nice to get iOS 8 to be able to turn this off.