A GP probably has that proprietary Raleigh 26 tpi threading, so if you get a replacement fork that's not from a Raleigh GP, you'll have to get a new headset, or at least the upper race; or maybe upper cone and race. Were it mine, I'd just replace the whole headset. For a new fork, know your head tube height, plus the stack required for your headset (including spacers, cable hanger if centerpull brakes, etc.) -- the sum of these is the length of steerer tube you need. Can be too long and made to work, but not too short!