If you can get it for $100, then go for it.
But I thought your grandpa was trying to get rid of cars on the property, not add more?

Isn't that why you got the GT, to get it out of his barn?
Since you will presumably always be short on funds (a teenager with little or no budget), then I think your future holds a lot of bodywork on junky GTs. So, no point in avoiding it, get crackin'.
FIRST... make sure your GT is safe enough to drive and runs reliably. I'd patch the holes and rust spots. Then consider doing upgrades like a 4-speed. Don't give in to the temptation to take apart functioning systems, or you're backsliding from Car and toward Lawn Ornament again.
Autos are so rare, that any GT you do buy is likely to have a 4-speed in it, so it's not much of a luxury. When you buy a car, buy one that'll be the one you fix up first. Don't start accumulating iron on someone else's property.
Right now you don't put a lot of time into your Opel, so, careful about adding a 3rd GT to your grandpa's property. You already don't work on the one/two you have much at all.