British slang, interjection, exclamation of surprise short for "God's truth".
From Monty Python's Flying Circus Episode 1: Whither Canada?
Mozart: Thank you, Eddie. And now time for this week's request death. (taking card off piano) For Mr and Mrs Violet Stebbings of 23 Wolverston Road, Hull, the death of Mr Bruce Foster of Guildford.
Cut to a lounge setting. Mr Foster sitting in chair.
Foster: Strewth! (he dies)