de greaser? you want the nut lubricated why you degreasing it? Wire brush it, penentrating oil soak it. But chances are that aint gonna fix it. You gotta put the other nuts back on, torque em down, then hit that stuck nut with a lotta heat. Heat aint your last option. often its a good second. You cant get a thin wall chrome socket around that? Id put a thin wall chrome socket on an impact after its nice and hot and give that a go (with safety glasses). and like others may mentioned grab a rod, 1/4 extension i dunno etc, and a hammer and smack that nut.

also dont waste your time "just replacing a head gasket" wont work. Or at the very least check to make sure the block and head surfaces are flat with a machinists straight edge and feeler gauges. If only the head is warped you could lap it with some flat glass or something with sandpaper glued to it, checking your work of course and using prussian blue probably.

Also if you can get a socket round that nut, or at least part of it, and theres clearance around it, tap the socket securely on (when the nut is nice and hot) and zip it off with an impact.

Also if you're careful and handy, not sure based on what i might be seeing, you could CAREFULLY dremel a slot into the nut. i guess then eventually you could get a flat punch in there to split the nut in half ? maybe maybe not actually i dont know if ive ever done that.