Green’s Dictionary of Slang

guts up adj.

also guts high
[gut n. (2a)]

(US) fearless.

[UK]R. Frede Entry E (1961) 55: Stew Trent, the sometimes wrestler, but all times drinker and whorechaser and general guts-high gung-ho aborigine.
[US]W. Murray Sweet Ride 206: Denny Maguire? He was guts up, man.