A GEM (Game Extension Module) was one of Monte Cristo's original concepts for developing the game.
It was never completely clear how exactly the GEMs were supposed to work, but from MC's blogs it became clear than they were more or less to be a tycoon-style mini-game, sold separately, like a mini-expansion or a plug-in for the main game. In the Planet Offer online interface there was a description category for each city: 'GEMs installed in this city', which suggests that after buying it you would have been able to activate it in a city, or not. Possibly, activating a GEM would have changed gameplay in this particular city.
The main idea for the actual mechanics of the mini-game was oriented towards concentrating in a particular resource, and developing it in much more detail, than it was possible in the basic game. For example, the first GEM planned, and almost completed, the Ski GEM, was to enable you to build ski resorts, complete with ski tracks of different difficuly, running down the snowy slopes of the many mountain maps of the game. In case your resort were successful, it would bring in a large amount of tax revenue, allowing the city to sustain itself exclusively from this activity.
Other GEMs that were planned were the Beach GEM, dealing with running a sea resort (this was to be the second GEM, and as such was developed also quite extensively), an Airport GEM, dealing with running and expanding airports, and an Industrial GEM. Other files in the code suggest also other Transport-oriented GEMs were planned.
Unfortunately, Monte Cristo's bankruptcy put these marvelous plans on an indefinite hold. The only development, seen to date, is the inclusion of the majority of the Ski and Beach buildings, with limited functionality, as the Mountain and Beach packs in Cities XL 2012.