I never heard of a gasket on the finished side of a tub overflow. Afterall, the purpose of that fitting is to oveflow! You put the gasket in the right place. It should have been a tapered thickness, to account for the slope of the back of the tub.
The shoe gasket can be replace by unscrewing the flange. The old gasket can be removed and a new one installed throught the hole, from the top. They make some tools to unscrew this flange if you need them. Available on the net or from a good plumbing supply.