Okay, learning is supposed to work this way (see MM7, for instance).LS = learning skill level
XP =
none: 1 * raw XP
normal: (1 + 0.09 + LS*1) * raw XP
expert: (1 + 0.09 + LS*2) * raw XP
master: (1 + 0.09 + LS*3) * raw XP
GM: (1 + 0.09 + LS*5) * raw XP
The first three (normal, expert and master work correctly [as in MM7]), but GM works something like
GM: (1 + 0.09 + LS)*5 * raw XP
In other words, the right parenthesis got moved over two characters in the formula. This obviously smacks of a bug that's really just a typo.
So again, with GM you DO get a big bonus, even the "correct" way, it's just not a ludicrous bonus.