Was curious if there was any interest in this game. I brought it up a couple years ago. It's not like the sim leagues per se. Kingdom, Hammer, Witlon, Deez, and Schisno play in it. It's pretty difficult to explain, but basically:
What it is: team and roster construction, drafting, signing free agents, hire and fire coaches. Like the NFL offseason essentially. The game is played on a forum, with posts and stuff, not like OOTP. It lasts for a few months of the year, every year from FEB to JUN or so.
What it isn't: a continuous sim with games played and winners and records (as of now). We used to sort of do it on Madden but it became a pain.
Winners are generally determined by vote. So it really depends what one wants. If the sim leagues are fun solely because of competition and such, GML may not be so fun. If part/a lot of the fun for the sim leagues is trading, drafting, signing and building your team (which is the main purpose I play, I've stated I have winning low on my priority list overall) then you'd enjoy the game
I'm not sure if that fully sums it up, so if there are any questions, please ask. Here's the league from a couple years ago for an example. GM League: Year 1 - Stadium Rejects