FGS (Future Game Simulations) is a model based on the ICM model, which is designed without most of the well-known limitations of the ICM model. To name a few, FGS takes into account posting the blinds in the upcoming hands and the positions of the players at the table.
To learn more about the FGS model, read these articles:
Currently, ICMIZER 3 allows using the FGS model with depths 1 through 6.
With FGS 2 it looks 2 hands ahead, with FGS 1 it looks only 1 hand ahead, so it is faster.
The recommended depth setting is the number of players at the table minus one, making it FGS5 for 3 players, FGS3 for 4 players, FGS4 for 5 players. With more players at the table FGS model becomes very slow, so not all FGS depths become available.
The recommended setting is at least FGS2 for situations with more than 5 players.