Gamma GT seems quite an unusual test nowadays, and seems not to be listed on "standard" LFT requests.
(Perhaps since we could buy 24 cans of Fella's 'R' Twoits for a tenner at Tesco's.)
Never come across it on this forum, but think the Courts, (and sometimes the patient) asks for it when they are trying to retrieve their driving licence after drunk driving- seem to remember 2 bans in 10 years triggers a test.