Doesn't GMF work on elemental's as well??
I have absolutely no idea, but guessing since it supposedly only works on animal companions I'd say probably not.
Pretty sure elementals fall under the same category as summons, since they use the same combat mechanics as those.
Think it doesn't work on familiars either, but I can't say I've seen rangers / druids combined with sorcerer / wizard levels before.
But if I had to make another guess it's that this domain is mainly designed towards clerics that multiclass into rangers.