GPC/DJGPP is a DJGPP V2 application, and most of the DJGPP documentation applies for GPC too. A great source of information is the DJGPP FAQ: http://www.delorie.com/djgpp/v2faq/230b.zip
Another place to look for DJGPP documentation is the DJGPP Knowledge Base, at this URL: http://www.delorie.com/djgpp/doc/kb/