I own a '94 pontiac GP 3.1 (120k) and have noticed oil in my coolant. Does anyone have a step by step guide to help me replace the intake gaskets? thanks.
( should I buy a dealer gasket set or aftermarket?)

Fel Pro gaskets seem to be the best.

The link below would be the best place to look for a write-up, although it's a pretty straight forward procedure.
http://www.60degreev6.com

I just had to replace my intake gaskets on my 95 grand prix (3.1) cause of my coolant leaking out of the sides of my motor. I used factory OEM intake gaskets. Thats the only intake gaskets that I would use. Haven't had any problems and they fit in there nice and snug. I wish I had a step by step guide for you but my brother inlaw did the procedure for me. I would go to Pep Boys and buy a Haynes Automotive Repair manual for your grand prix.