'General Practitioner' (GP) is UK-terminology for a primary care physician ('family doctor').

GP can also refer to General Practice, the remit of a General Practitioner.

Glyceryl trinitrate (GTN) is a medication used in the prophylaxis and treatment of angina and the management of anal fissures, amongst other indications.

NICE - GTN

Genitourinary medicine (GUM) involves the investigation and management of STIs and HIV.