I'd say that it isn't an asspull, at least in Luffy's case. Gear Third is hard to see as an asspull. He could already inflate himself with Gomu Gomu no Balloon, so it's not to much of a stretch for him to inflate his bones. Gear Second is harder to debate, because I don't know enough about anatomy, but I feel like the explanation of the mechanics was definitely good enough to let it fly.
I can't deny that Asura, Zoro's power-up, was an asspull, but I feel like it could easily be retconned out of being one. It could just as easily turn out to be a unique way of manifesting Haki as it could be an asspull. Oda hasn't elaborated on it yet, so who knows what it actually is.
Sanji's WAS an asspull. I mean, one could argue that it might've been subconscious use of Busoshoku or something, but HE LIGHTS HIMSELF ON FIRE USING FRICTION, AND SOMEHOW USES IT UNDERWATER AFTER THE TIMESKIP. I can't even sell myself on it.
But I just want to point out that at Galley-La, they didn't have the same motivation as at Enies Lobby. At Galley-La, they didn't know that Robin was being blackmailed by the Buster Call, or that she was sacrificing herself to save them. After gaining some insight and fully understanding where they stood, they could go all out to rescue her. Simply put, they didn't hesitate to save a friend in need.