Head gaskets are not common on a 3100/3400 unless something else went wrong (eg. L36 Killer's water pump issue). Regardless, as Scottydoggs suggested, I'd get a compression test done first to rule it out completely. If everything checks out with that, I would say there's a 99% chance it's the LIM gasket, which almost any GM motor from the '90s and early '00s are notorious for. I recommend getting an intake gasket kit which should also include valve cover gaskets, EGR gasket, etc. If you're in that deep already, might as well change everything out. It won't cost much more. Here's an excellent video on how to do the job with plenty of details.
Intake Manifold Gasket replacement GM 3100 V6 95' grand Prix - YouTube
