Define "fixed". Nothing more than 30 years old is ever completely fixed. Well, maybe me, after the second vasectomy. But if the basics are attended to, they are quite durable cars.
The engine is very robust, with many doing 100,000 miles or more without ANY mechanical repairs. And a top to bottom engine rebuild can be done for under $2k. Many are under $1000, if you do the wrenching yourself.
The weak spots are quite well documented. The stock Solex is JUNK, easily replaced with a Weber.
The electrical system is fairly decent, but the headlight wires MUST be replaced ASAP if they have not been already (and right up to the junction in the wiring loom near the middle of the engine compartment; the forward wiring is insulated with rubber, which has rotted. Guaranteed). The rest of the electrical has a few weak spots (ignition switch is weak, fuse box terminals can get corroded, turn signals have a ubiquitous glitch where you get a double flash, but all easily remedied).
The stock ignition system uses points and a condenser, but electronic systems are available for less than the cost of a tune up, and once installed properly, will provide trouble free operation
The headlights can give some problems, but so long as the PO (Previous Owner) hasn't hacked them too bad, they can be made to work very well.
The brakes can be totally overhauled quite readily, and although not up to modern standards, are quite good for a 2000 pound car. Bigger, better brakes are available, it just takes money.
The suspension bushings may need replacement, but parts are readily available, including polyurethane bushings which will make it handle better than new, and for longer. Also front and rear sway bars are HIGHLY recommended.
The body is prone to rust in a few locations, even in a dry climate (battery box being the #1 culprit), but the repairs are also easily made if you start with a relatively rust free car. But if it starts off as a rust bucket (which describes MANY, if not most North Eastern GT's), it will not be cheap to repair, and rust will continue to re-appear. Washington area cars are usually pretty good (my SportWagon was from Tacoma, and is virtually rust free. Except the battery box), but watch that the GT you are looking at didn't spend some time in the rust belt.
Other mechanical systems are decent. The auto tranny is a standard GM TH 180, and very robust. The 4-speed is solid, but prone to worn synchros. But they are easily rebuilt, or replaced (lots of used trannies out there), or even better, upgraded to a more modern European Opel Getrag 5-speed.
There, that about covers it. I'll move this thread to the "GT" forum, since it is heading in that direction (it fits my criteria my criteria for being "GT Specific")
HTH