Green’s Dictionary of Slang

gallop one’s antelope v.

to masturbate.

1938		J. Curtis They Drive by Night 52: You must have been galloping your antelope, I’m thinking.
1966		Trimble 5000 Adult Sex Words and Phrases.
1972		R.A. Wilson Playboy’s Book of Forbidden Words.
1980		Maledicta IV:2 (Winter) 192: We also come across specialized terms in the male masturbation phrases such as flog the bishop, gallop one’s antelope.