well, geometry wise you have only a few things to mull over. The dropout to fork crown distance can vary a small amount and that can have an effect on what rubber you can fit in there, and thereby, then handling.
then there's fork rake/trail. Some variance possible there, depending on the designer. Finally there's always the length of the steerer and whether it will fit the headtube/headset combo. That a fairly easy straighforward measurement.
Give all that it's likely the fork length and trail will be very, very close on most GT bikes, even those with some years separation. Bike geometry is generally pretty close. But there's always the possibility of some signifcant variance. I'd still be that yo uhave a better than 90% chance that the old GT Rigid fork will work fine on your frame.
Steerer length? measure the headtube and factor in a stack height for the headset and then make sure you don't go shorter than that measurement + 1.25 inches (for stem attachment) for the steerer.
And if you're not too concerned about not having disk brake mounts, then you'll prolly find this will work fine.