Green’s Dictionary of Slang

a fat person, also as nickname.

1965		(con. WW2) S. Lassen Anders Lassen, VC 160: We had a small pig we called ‘Gutso’.
2002		Harold Hark Moon Food Cafe Ch. 19 🌐 Instead of a cigarette, I’ve got a Gutso out there who’s gonna cream my ass.
2015		T. Weston Boy Who Lost His Name 20: That’s when it became Fatso, Fatso, nothing but a gutso. They seemed to think he could simply stop the one thing that he was really good at—eating.