What headers am i able to put onto a non-supercharged 99gp gt?

All the headers that fit SC also fit your NA. It'* the same block, same emissions equipment and same engine bay. Works out nicely as it'* all interchangeable.

Personally..if depends on if you want low cost with good gains or higher cost with perfect fitment.

i want some pretty good gains aswell as good fitment. i had my mufflers deleted but other than that im stock. do i HAVE to get my entire exhaust system redone if i get some preformance headers?

Look at speeddaddy headers. Good price, and good headers. Bolt right in and no need to modify the exhaust. You will have to hit the firewall a bit with a hammer to make room for the O2 sensor, that'* about it. And will need a O2 sensor wire extension.