The cls_spgn_*.2da files are the .2da files that define how many spell slots a class has at each level. (The "gn" is short for "gained", but it is the total number of slots that is defined, not the difference from the previous level.) These files are used only when referred to by the "SpellGainTable" column of classes.2da. These .2da's have 60 rows (numbered 0 to 59) to accommodate non-player characters (who may exceed the player character limit of 40 levels). These .2da's additionally have a blank row between the headers and row 0.