It seems to be a Victorian term for cunniligus:
I was sliding the final drawer back in the desk when Ray asked, "What the hell does gamahouche mean?"
I made him spell it, then took the book away from him and looked for myself. "I think it means to go down on a girl," I said.I find it on the web at this Fark discussion, the Urban Dictionary of all things (it's usually a source for invented silliness or poorly written insults), the Wiktionary, and this rather steamy Victorian novel.
I shall endeavor to insert this word (heh heh) into modern parlance as often as mores allow.