If just using a silicone sealant, I would use 3M 5200 it's fuel proof. I would also consider using blind holes for the threaded part. Since the tank is inside the car you don't want to take a chance with fuel wicking up the threads.or silicone seal.
As far as I know, there is no silicone RTV sealant that is impervious to gasoline.
From the data sheet: "3M(TM) Marine Adhesive/Sealant 5200 is not recommended for use as a teak deck seam sealer. Extended exposure to chemicals (teak cleaners, oxalic acid, gasoline, strong solvents and other harsh chemicals) may cause permanent softening of the sealant."
Yea ok, why don't you actually try the product before condemning it?? They the MANUFACTURER say they don't recommend it, but they don't say absolutely not!! They say it may soften I'm contact with gasoline, but not that it will. Might loose a bit of durometer, but doubt it would ever loose seal.. think about how much more abuse it takes on an outboard (2stoke) crankcase.Your car; your choice David.
I would not use it in direct contact with gasoline; especially in this application. If that seal fails in the GT, it will be putting gas fumes inside the car. Most cars and trucks, the tank is more isolated from the inside of the vehicle.
Besides, the MANUFACTURER advises against use where it will be subject to extended exposure to gasoline, among other things. Better off using something that IS recommended for the application.
Just thought the OP should have all the facts.