I've been reading some Korean War-era comic books, and the term "gyrene" (in a couple of different spellings) has come up. The only online reference I can find is Urban Dictionary, which lists gyrene as a portmanteau of 'GI' and 'Marine'. Is this accurate?
Gyrene is UK slang for RM, portmanteau if you will of Royal Marine and Margarine.
There are lots of dictionaries on the internet - e.g. Merriam Webster, which says "origin unknown. First Known Use: circa 1894", and the Free Dictionary, which cites the American Heritage Dictionary of the English Language ([Perhaps alteration of GI + (ma)rine.]) and the Random House Kernerman Webster's College Dictionary ([1920–25; GI + (ma) rine with altered sp.]). If you want to dig further, you might poke around Google Ngram Viewer or ask in /r/etymology (a sub dedicated to the study of word origins)