|It's probably a 450MHz P3. |
First-gen P3 systems used the 440BX chipset. While many enthusiast BX boards could handle 133MHz, even 150MHz bus speeds, a Gateway BX board would be limited to 100MHz.
The fastest 100MHz FSB processor Intel made was the 1.4GHz Celeron (Tualatin core). You'd need a Slot 1 to FC-PGA2 adaptor to run the Tualeron on your board. However, the 1GHz/100 Pentium III (Coppermine core) would probably be a safer choice. I doubt that Gateway released a Tualatin-ready BIOS for that rig.
Super 939 | Dual-core Opteron 185 @ 3.2GHz | 4GB PC3200, CL2 | 1.0TB
2x GeForce 8800GTS in SLI | Blu-Ray | X-Fi Ti Pro PCI-E | W7 Ultimate x64