Green’s Dictionary of Slang

(Scot.) tough, ruthless; plucky; of clothing, ‘sharp’.

1973		(con. mid-1960s) J. Patrick Glasgow Gang Observed 103: He’s only five fit somethin’ but he’s the gemmiest – aw these great big blokes wir aw around him.
1985		M. Munro Patter 28: Gemmy is similar to the English ‘game’ in the sense of being plucky, and like gallus it can also mean sharp or flash: ‘Ye should have heard him givin up cheek tae the bouncers, he’s that gemmy!’ ‘Who’s the big yin wi the gemmy waistcoat on?’ A gemme kid is a young person who is bold, cheeky, sharp, cool, or any permutation of these.