GMC (actually Chevrolet division) would be the chassis maker - to the best of my knowledge they did not make the RV. I think Four Winds is the manufacturer, and Four Winds is now part of Thor Motor Coach. Chassis parts are available on the aftermarket - there are literally about a million Chevy Chassis motorhomes in existence. However, GM sold off the RV chassis business to Workhorse in 1999 and Workhorse is now part of Navistar. They do not make repair parts for older chassis.
As for the coach itself, no RV manufacturer makes or stocks parts for a 20+ year old RV. Most of the appliances and systems are industry standard components, though, and parts or similar replacement units are generally available.
For a rig that old, condition is everything. Check everything out thoroughly before buying. There are RV buying checklists in the forum Library - I suggest you use them.