If it's really stock not a thing done to it,
1. Fuel Pump, adjustable regulator, fuel pressure guage, hot wire fuel pump. (needed to run after market chips and prevent detionation)
2. Adjustable wastegate, boost gauge.
4. MAF pipe and open (cold air) system
and the list goes on and on gntype.com
you can spend weeks there.
#1 Dextron 2or3, F type will firm up the shifts a little.
#2 Don't get carried away with performance parts it may slow you down. Get the car running the best you can with what you have. Many Stock GN's run low low 12's.. You may have to change worn out parts like fuel pump or valve springs. Spend your money on a scantool and boost gauge first.