GSM with Vietnam Clasp seem to have been Australian personnel only…
South Vietnam
This clasp was instituted by Royal Warrant dated 8 June 1968 for award to Australian personnel. Only 68 clasps were issued, and all 68 went to members of the Australian Army Training Team.
The various qualifying periods, between 24 December 1962 and 29 May 1964, were
30 days' service in ships operating in inland waters or off the Vietnamese coast.
1 day in the service of a land unit.
1 operational sortie.
30 days' service on an official visit.
For service after 29 May 1964, the personnel were awarded The Australian Vietnam Medal.
http://www.stephen-stratford.co.uk/gsm_1962.htm