Gateway P-6860FX Core 2 Duo T5550 1.83GHz , looking to upgrade processor, need some advice?

No. The P8400 is definitely not compatible with your Gateway. The P-series of intel processors uses Centrino 2 technology, which uses a 45mm process vs. the 65mm process (which your centrino processor uses) - you won't even be able to fit the processor into the slot. If you want to upgrade, make sure the intel processor you're getting has a "T" in front of it.