Castel dual flush one piece toilet - Kohler - Toilet - One-piece - Gabrielle - K14257-WS-47

SOURCE: Kohler one-piece has no flushing power

Don't flush more water! Trash the toilet and replace it with a toilet that uses less water and flushes efficiently; practically every manufacturer makes a dual flush or eco model. Spend your money on a good toilet rather than paying for and wasting water every day.

SOURCE: I have to replace the gasket which the flush valve

The top of the pipe above the flush valve is threaded if you unscrew this you will be able to remove the flush valve and replace the gasket. I am assuming we are talking about the same here is a link that has all the parts:

http://www.plumbingsupply.com/kohlertoiletparts-santarosa.html