Mine had a chain too, but whenever I went to a "We Serve" gas station, the attendant wouldn't know the chain was there, and the cap would get pulled out of his hands and it would bang against the fender, chipping the paint. I did that myself several times, so I took the chain off.
If you still have the original rubber flap that would hang over the fender when you open the gas cap door, then that should protect the paint at least...
Nigel
'73 240ZT