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Grace actually calls Gabriel that. Sort of. In GK3, there's a point in the game where you can make Grace go to Estelle and Lady Howard's site (How she knows where that is is another question altogether.). If you look at Lady Howard or Estelle, Grace thinks "Lady Howard is a slave driver! I'd hate to be in Estelle's position. Hey wait a minute!" What's up with the "Hey wait a minute!" part? In what way is Gabriel a slave driver?
I'm rather late to this thread, but if I may nevertheless respond:

It occurs to me that Grace may have been referring not to Gabriel but to the player--the person, after all, who has been commanding her (and Gabriel) all over the French countryside with barely a rest.