Fantastic.
We have followed the steps you outlined, and everything is now working exactly as intended. Thank you for your quick, concise, and useful feedback. I will mark this issue as solved, but leave it up for anyone else that experiences the same problem.

Cheers,
Jazzpants

Hopefully someone has encountered something similar and has some insight into the problem our game is facing. I am posting here because there are no forums on the GMR site yet, and I didn't see an answer to this in the FAQ section.

I started a game with G&K and BNW, but none of the other DLCs as I got the game through the humble bundle. I specifically hosted the game because I had the least amount of content compared to the other players (who all have the Gold Edition), and didn't want to run into the "missing DLC" error. However, one of our players is getting that exact error, and we cannot pinpoint what DLC he is possibly missing. My enhanced steam is telling me that everyone in the game definitely has both expansions, and two of the players with the Gold Edition have already taken their turns.

As far as I can tell, all the players have all of the required content. Is this a savefile error, or perhaps something with his install? He is also reporting that when he goes to check what DLC he has enabled, G&K becomes unchecked whenever he starts a game; this seems like unintended behavior.

Any advice would be much appreciated

Civs in game: Arabia, Rome, Maya, Persia, Byzantium, Poland, Brazil, France
Host DLC: Vanilla, G&K, BNW, Conquest of New World Deluxe, Genghis Khan's Mongolia. Game was started with 3 expansions, no DLC
Everyone else has Gold Edition.

Cheers,
Jazzpants