As the author of The Winner of Sorrow, I remembered the word from my teenage years in Dublin (that is, in Ireland). In those days gamhuche meant fellatio - not that we knew what fellatio was, or cunnlingus for that matter.
"'You're not selfish, Mister C; you're bothered, that's all. Think about money, think about happiness, think even about me. They're prizes worth having, ain't they?' She kissed him on the cheek, then broke away to continue her inspection, opening the door to the bedroom. 'Hey, lookee here! The monk's cell. Is this where you gamahuche the boys?' He didn't know what the word meant, nor did he want to. He waited for a moment, then went in after her. She was sitting on the edge of the bed, head bent, hands in her lap, and when she spoke her voice was different." -- The Winner of Sorrow by Brian Lynch, pp 40-41
ETA: "…the word might come from Gamahut, assassin guillotined in 1885; but, besides the date, nothing legitimizes this etymology which is quite doubtful. Most likely, it comes from gama-ut which means “the lowest sound of the scale�?, from which one could infer a verb gamahuter with the meaning “to go down (going from high to low pitches).�? However, it is truly the idea of “to go down�? that is at the base of the synonyms … As for the form gamahucher, it could be the result of crossing with hucher “to yell, to call out loud.�? Also see gabahoter. In modern usage, gamahucher is used most often for cunnilingus.
-Giraud, quoted (and translated) by Seth Adam Whidden in “Leaving Parnassus: The Lyric Subject in Verlaine and Rimbaud�? (source)