Depending on what year these toilets were made, different tank flappers were used. The past five years Gerber, like many others were working on the perfect 1.6 toilet. Each had a different tank flapper. Since, what you are describing as your problem is the flapper, that is all that it seems needed to repair the toilet.

Fluidmaster makes the adjustable dial flapper which will replace your current one with no guess work as to the time needed to stay open. The longer the flapper stays open the more water flushes. Put this in and start at the first setting and adjust if needed to lower consumption. This also contains a kink free chain as well. Good luck and May the flush be with you.

may want to try a universal flapper that has a solid rubber "chain". this inserts into the holes in flush handle arm. can find them at just about any hardware store.