Your RP was arguably worse than his.

Were his actions childish? Absolutely, but he's roleplaying the equivelant of a Gnome teenager. Depending upon his role, that isn't necessarily a bad thing. You honestly have no idea what role he's playing.. maybe it's a brain-damaged Gnome with an obsession with his own mother? Maybe he's just a prankster teenager that's trying to rile you up. Both are perfectly feasible roles.

However, you clearly were not roleplaying at all. Using multiple question marks isn't a roleplay device, it's OOC frustration/annoyance bleeding into the game. And you telling him he has proclaimed wisdom? Did he ever proclaim it, or are you basing that on what you know to be his wisdom stat? That isn't exactly stellar RP. Does your character somehow know more about Gnomish culture than an actual Gnome? Maybe you could say "Hm. And I thought all Gnomes were mature and wise. I must have been wrong." That would make more sense, IC.

In the future, it is never appropriate to challenge someone else's roleplay IC. It just isn't possible. You don't know what role he's playing. Just role with it as your character would.