Game System files are specific to the rules for a gaming system, such as 3e, Unisystem, D6, etc. These files define what rules apply for that system, such as Hit Points, vs Vitality Points, Armor Class vs Damage Reduction, etc.
There are five files in a Game Mode, they are:
level.lst
load.lst
miscinfo.lst
rules.lst
statsandchecks.lst