Being a Gateway special board it is hard to find. So probably a higher Pentium D 9xx series is max but can possibly be confirmed on Gateway's website putting in your computer model number.
My personal opinion with hardware that old is that it is hardly worth the effort and money to upgrade.
Got to agree.. I wouldn't bother faffing around getting an out-of-date chip for an even older motherboard. If you're needing an upgrade, I suggest a new build.. for a couple hundred pounds you can have a great motherboard with onboard HD graphics, sound etc and a good dual or tri core chip. A new case, RAM etc.. every could be done for very little now a days.