Which Google apps can be deleted or disabled without harm to the system?
This very much depends on the device and the Android version running on it, plus where you've bought it – as the lot of pre-installed apps might differ based on that. I'm not sure about Gmail – but the other apps mentioned (Drive, Hangouts, Play Music/Books/Games, Google+) are the first ones I always disable, and never had any side effects. At least Music/Books/Games and G+ should be even safe to remove. You can also deactivate/disable the contacs sync adapter and calendar sync adapter if you don't plan to sync your contacts and calendars with Google.

What you definitely should not touch is the Google Services Framework – or you'll break "all things Google" on that device, including the Playstore.

If you want to get rid of almost all the crap, and don't need the ability to buy apps on Play or use apps that check their licenses against Google Services, flash a custom ROM and microG, plus install either Blankstore – or, if you need to buy apps and check their licenses, the original Playstore app. Details I have described in my article Android without Google: microG.