I doubt it's a very difficult thing to write, code-wise. I mean, you could even do it yourself, if they'd written the utility for converting a game save to a map save that they had in vanilla Civ IV. And it would have a such an awesomely positive impact on the game's multiplayer...it's a real shame.

I'm organizing one game right now, setting it up so that all of the players are online at once once a week. People dropping shouldn't be an issue, but if tedium might be.

Two follow-up questions:

First, so with eight or twelve players, all on at once, it's not that bad?

Second, are there any scenarios you would suggest for later starts that give you more to do at the beginning of the game?

Ideally I would want to play a standard concurrent-turns multiplayer game with friends, then save the file and strip out a scenario map once it's sufficient advanced that I can plug and play in GMR. Unfortunately that isn't possible because stripping a map out of save is not a feature that was ever written for Civ V (it was last available in vanilla Civ IV). What a boon it would be if such a thing were possible. Alas.

I can see GMR being incredibly efficient late game overhead-wise, but don't want to use it in, say, the ancient era, when every turn I make one or two moves and then have to save, upload, etcetera etcetera. So people that have rather more experience with this than me, how much of a pain is it to use GMR when you're just starting out?