Green’s Dictionary of Slang

a generic term for any very distant place.

1881		Times 23 June 4/5: Having [...] docked the steamer they cleared out ‘for Guam’ .
1885		Bulletin (Sydney) 11 July 20/1: When the case was called, was he there? Was he mad? He had disappeared – vanished – gone to Guam. (This, by the way, is a pleasant mercantile fiction of Melbourne shippers. When it is not deemed desirable to state what port a ship is bound for, they run up a flag with G.T.M. – gone to Guam – on her masthead. Guam is a little uninhabited island outside the Heads.).
1898		Westminster Gazette 7 Dec. 12/1: Sailed for Guam [OED].
1997–2001		Online Sl. Dict. 🌐 Guam n 1. any location far away. Subjective, usually used to refer to anywhere it would be a hassle to travel to. (‘That’s way out in Guam!’).