Can the overflow that holds the flapper be removed and replace the rubber gasket under it?

yes on a toilet I am guessing .. you can take the tank off the bowl and unscrew the nut of the overflow and replace the rubber seal. To make sure you wont get mad if it leaks after you put it all together is run a bead of CLEAR silicon around the new rubber seal and around your tank washers before you put it together and it will seal up any tiny holes or misalignment you have.