OK this is what I can tell you from personal experience. The loom is the same except for the bit behind the actual guages. You need a GS/GT loom with the extra wiring and bulbs for the oil pressure, amps, fuel, temp. A half decent sparky can mod a standard loom to work. The fairmont loom ( well my fairmont loom anyway) has an extra wire going out near the coil. Turned out to be the tacho wire so that made things easier. You'll still need a different oil sender and temp sender otherwise the guages won't work. I got a GT oil sender and ended up using a temp sender from a ZH fairlane I was using to supply the donor running gear. It worked fine. Did you get any of the wiring with the GT dash? At least the plug that goes between the back of the speedo and tacho?

If you got the plug and the first couple of feet of dash loom going down to a BIG plug you could just about do it all yourself. If you just got the plug and a couple of inches of wire
find an "old time" sparky and he'll set you right. Have you got a spare wire sticking out of the loom at your coil? Some cars with the standard dash did, some didn't. I was lucky. Oh one other thing. These looms are old so put a couple of relays in. Especially for the headlights. You don't want a dash fire, they aren't nice.