Question:
"I do wonder what the difference is between those for whom this occurs, and those for whom it does not."
Answer:
This has to do with the system clock time setting on the physical server hardware on which your synced account is located. The "GMT Bug" is picking this up from the server and then transposing your appointments into whatever time zone the server's clock is set to. If the server is set to GMT then you'll see "(GMT)." If the server's clock is set to PST you'll see "(PST)" and so on. If your server's system clock is set to a different time zone that you are in, then you will see this bug manifest on your iOS device(s). If it is the SAME as the time zone you are in then you won't see the bug, but the bug is still there in the code on your iOS device(s).
Here's an article by AppRiver, my MS Exchange host who has spent tons of time working on this both with me and other users:
http://blog.appriver.com/2014/12/ios-8-calendar-events-display-dual-time-zones/
This describes what happens on MS Exchange servers but the same exact thing happens on Google servers.
Others in this thread posted about how stunned the Apple Store employees were to open up a brand new iOS device in the store and see this bug show up without any changes whatsoever being made to default settings. This bug is embedded in iOS itself...every single device running iOS 8 has this bug embedded on it. The only people who will experience the monumental hassle it creates are those who sync to accounts on servers set to a different time zone than their device time zone. There are people out there right now running iOS 8 that don't see this bug. But as soon as either 1) they travel into a different time zone than the one they are in and their iOS device updates its clock; or 2) their account gets migrated to a new server with a different time setting (happens as part of automated load-balancing operations by providers like Google), the bug will show up....and they will all start posting here stumped as to how this happened without them changing any settings.
I promise.....follow this thread (or go back through it) and you'll see it. Every few weeks when the Google servers migrate a new group of accounts around, a bunch of new people show up....don't read any of the previous posts in the thread...and start saying that this "must be due to Google because Apple says it is" and the fact that they didn't change any settings on their device "proves" this. As explained above, this is already on all devices running iOS 8 and you don't have to change any settings to encounter it...all that has to change is the system time setting on your synced account server. The bug was there all along. It's like a leak in your roof: it could be there for months, but you aren't going to know about it until it rains right?
Thanks for taking the time to call in and report this to Apple. That really is the most effective thing you, or anyone can do. We need to register as many cases of this issue with Apple as possible so that they make it a priority.