does anyone kno if a 3800 series 3 motor will fit in a 1999 gp gt and if so wat all will i need to make the swap?

I know you can do it but you'll have to wait for those more knowledgeable to answer your question in full. I think you need the PCM out of the donor vehicle for a start

You'll need to convert it to your 3 wire coolant sensor, use a stock MAP, an l36 fuel rail and a TB adaptor to make your tb work.

I believe that's it.

It is possible but a few accessories will need swapped like throttle body and a couple of sensors/wiring harnesses.