Replacing toilet tank gasket
That is a Kohler "canister style" flush valve. 

First, carefully remove the "rim-clean/bowl-fill" water supply hose (the little black hose that runs from your fill valve to the flush valve. Also, remove the chain that attaches the trip lever to the flush valve.

To get at the gasket in question (which is very likely, but not 100% certainly, your problem) you need to carefully twist the "small part above the white ring" counter-clockwise 1/4 turn. This will release the entire canister assembly from the base. It will look like this (note chain still attached in my pic):



The pink washer on the bottom just stretches and pulls on/off. Reverse previous step to reinstall...

NOTE- I have performed this job without removing the hose and chain, but it is easier with them removed.

screen-shot courtesy of REVIEWS@ca.rr.com