Green’s Dictionary of Slang

1. a lame person; also as nickname.

1889		Notts. Guardian 2 Feb. 3/3: William Hollis, alias ‘Gammy,’ hawker, cjharged with having wound ed mary Ann Moore.
1934		D.L. Sayers Nine Tailors (1984) 244: ‘Gammy Pluck,’ observed Mr. Parker, distantly.
1948		Gloucs. Echo 17 Feb. 4/3: The highest individual score of the evening was 133 by ‘Gammy’ Roberts.
1984		Partridge DSUE (8th edn) 445/1: late C.19–20.

2. (Aus.) a fool.

1892		H. Nisbet Bushranger’s Sweetheart 191: Well, of all the gammies you are the gammiest, Slowboy, to go and string yourself to a woman, when you might have had the pick of Melbourne.
1945		Baker Aus. Lang. 130: Fools of one kind and another [...] possum, gammy, gazob, gimp and gup.