Green’s Dictionary of Slang

guntz n.¹

the supreme example, the best.

1959		F. Norman Fings I i: He finks he’s the guntz and he ain’t.
1962		F. Norman Guntz 32: In fact all the way round he looked the guntz.

In phrases

go the guntz (v.)

to commit oneself, to ‘go all the way’.

1977

D. Powis Signs of Crime 187: Guntz, go the Go all the way (East End Jewish expression).