In addition to hand-picked elite MechWarrior candidates from various SLDF units, the Gunslinger program also accepted some of the top SLDF MechWarrior pilot graduates as well, including a young Aleksandr Kerensky. (Probably because they were less stuck in their ways and had less to unlearn.)
In addition to relearning how to pilot a BattleMech, the one-year program including training in various other fields such as Oriental Martial philosophies and advanced neural-thought technology.
In some ways it could be compared to the training received by candidates attempting to join elite special forces units such as DEST or Death Commandos (with their military training focused on their MechWarrior skills of course) in order to hone their skills to a razor's edge.