The gasket some are talking about is the flapper. the flapper is the seal. there also is base seal that the flapper seals against. again if water Not running down the overflow it 99.9999% it's the flapper (or Big Black or red Gasket as some call it). Very easy to change in about 20 seconds. Now it its running down the overflow tube either the float out of adjustment or the FILL VALVE won't seal when the Float arm puts enough pressure on it to close it. That takes about 15 to 30 mins. to change with some common knowledge of hand tools.
common toilet parts - Google Search
again it this the type of plumbing hardware installed?