I think Palpitoad gets Power Whip because of its tongue (which is still odd), but yeah Grimmsnarl gets it because it can use its hair.
Gyarados...idk. Either it’s using its whiskers (in which case why don’t Swalot, and Whiscash learn it?) or...its entire body. Which is hilarious to picture but...
I was really confused by it as well. I did some research and the best I could find is an article here about bag worms that states “The females produce a powerful scent, or pheromone, that attracts the males.”
But this applies to the moths that bagworms turn into, so yeah still pretty shaky...
I’m not sure if this has been mentioned yet, but a few I find odd at time of writing:
-Aurorus doesn’t learn Aurora Veil
-Ampharos doesn’t learn Tail Glow
-Emboar learns Scald
-Ludicolo learns Fire Punch