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Use one of those rubber strap oil filter pullers, to turn it loose, saves causing any damage, then check the condition of the cap gasket or corrosion on cap or tank lid, clean it all if necessary.
I am assuming that it just clicks and will not unscrew? You might have do what jeff78 said and destroy the cap by crushing the top in with large channel locks to where the slip/click locks up. If it is a replacement plastic cap, do that, as you can get another one. If it is an original metal cap wait and see if anyone has any tricks. mds...
You probably have to destroy it. Before you start get an old movers blanket and cover your paint. You will kill yourself if you damage any kind of decent paint. If it is non locking try a large set of vice grips on the handle and try pulling up as you turn it. If it is a locking type ram a screwdriver into the keyhole and then try to turn. This cap is probably junk anyway.
Never had the Vette do that yet but my Ferrari does this a lot, it will get so tight there is no getting if off, but if I let it set over night the pressure drops enough to loosen the cap. maybe wait and try this before you ruin the cap?