gas tank filler gasket

There are 3 "seals" that I know of.

1. Seal in the gas tank to the filler tube
2. Seal in the trunk floor to the filler tube
3. Seal on the quarter to the filler flange.

#1 has no screws

So I assume we are talking about #2 or #3

In general you don't tighten up A screw till ALL the screws are started. Maybe loosen the straps on tank a tad so it can move a bit.