I've never had a problem with calendar invites in other timezones on Google, and I suspect a number of those come in UTC. Is it possible that the UTC invites you are getting don't actually have the timezone attached? In the absence of a stated timezone I'd have thought assuming the user's current timezone was the safest course of action

You can't create a calendar invite in UTC. Period. This was a year ago; I don't remember the specifics, but either the invite I had was for UTC, and Google Calendar put it in in my own timezone, despite it being clearly UTC, or I had to enter it by hand, and there is no way to specify UTC/GMT by hand in Google calendar. The issue is not an import issue, it's that Google calendar literally does not have the option, at least that I can find. You have to select a region, and there is no special 'UTC' or 'GMT' region, and clicking on the UK does not give you "Greenwich" or "GMT" either.

Though per another comment, yes, if you happen to know a locale whose timezone is the same as UTC/GMT, you can use that, but that's still hilariously unfriendly to the user.

Without trying to diminish the problem, I'll mention that Google Calendar does support the Iceland time zone, which happens to be equal to UTC.