I don't know about gluing them. I've heard of body "adhesives" like that, but don't know anything about them. Then there's 3M tape, which works well for alot of stuff.... but the GT effects are kind of tricky. They are mounted at angles that tend to want to pull away from the tape and are VERY difficult to get lined up straight with the door moldings. On my old GT, one of the mounting strips was missing at the front of the side skirts. I used 3M tape to hold it on at the top, but it would always come loose and was impossible to get completely straight. I re-taped it many times before finding a replacment metal strip. Once I mounted it with that, it was solid and straight.
The strips I'm talking about are metal strips that mount with rivets along the rocker panel and at the tops and sides of the side skirts. The GT ground effects "wedge" tightly down into them, hold them straight and solid, then you attach the inner-fender and under-car places with screws. It's a very solid and well-designed system of mounting them. It would take alot to get them to come off.....like a wreck.......been there.
Have you ever seen a GT and the tops of the side skirts on each side of the door moldings were crooked from the door moldings? It's very common... I see it a good bit. I imagine it's either from years and years of vibration or, more likely, they were removed and not properly re-installed.
The strips are not hard to install, but are probably tricky to find. I had all of mine from my wrecked GT, so I didn't have to go looking for them. If you found them at a junk yard, they would probably be cheap because they probably wouldn't think they were anything important. I know Mustangs Unlimited has them new. They sell them individually, piece by piece. the guy you got the effects from doesn't have the car still? If so, you could get them from him.
Maybe someone who has attached ground effects in other ways will jump in here with some ideas. If you find the metal strips, I can tell you exactly how to mount them, with measurements, etc. Or if I can help in any way, let me know.
Hope this helps some!