right now my ga is up at pacesetter having headers built for it and i was wondering what type of gains im looking at with stock everything except intake

Slight, but some. Headers help but opening the valves more will make the biggest difference. Is the GA supercharged? Think of it this way. What goes in needs to come out. So the system can only move as fast as the slowest section. You should notice a good sound difference though. The tone should drop a bit deeper.

nope ga is 100% stock. only reason im getting headers is beacause i have a freind that works for pacesetter and he needed a ga to build there new 3400 headers on and the persont that loans there car gets the first set free

Can't beat that price, great deal. Can't wait to hear it. I'm sure there'* a few 3400s here that may be interested in a set.

I'm going to guess 15-20hp. You have to consider that with an L67, headers make 20-30hp with a smaller pulley and an opened exhaust. While 3400 heads flow better, the L67 is boosted. You'll feel it, but don't expect a miracle until you make a few more modifications.

wasnt expecting much

will be expecting something one the exhaust goes on a cam goes in and it has a tune

With an exhaust, cam, and a tune, you'll get some very impressive numbers out of it. The tune will make a huge difference.

You really don't add horsepower with headers, you salvage the horsepower that is already there but that had been held back due to exhaust back pressure. But, to get the true salvage from the headers, the rest of the exhaust system needs to opened up as well - HiFlo cat, resonator removal, larger diameter pipe and HiFlo mufflers.

Will the 3400 headers fit on a 3100? I don't know much about the 3100 but I have an interceptor 3100 in my Grand Prix and it feels faster than my SSEi and it would be awesome to build it since it needs a rebuild anyways...

They should fit, as they're both 60 degree V6'*. What year grand prix is this?

If it feels faster than your SSEi, then there'* something wrong with your SSEi.

3100'* in w-bodies run 16-17 second 1/4 miles.