I have a Toto UltraMax with Power-Gravity valve. It's about 3 years old. The flush valve has gone bad - it doesn't stop filling unless you jiggle the valve. So, I think it needs replacing.
Question: the Power Gravity valve looks very similar to pictures I've seen of the GMAX replacement valve. Can I replace the Power Gravity valve w/the GMAX?
The fill valves are pretty much universal (for most brands), so yes, I think it would work. But, depending on the valve that is there, sometimes all it takes is a new seal for a buck or so.
A completely new valve is less than $10, so it is your call. Many of the new Totos use the Korky Quietfill valve. If you replace it, make sure to adjust the water level per the instructions.
Important note - I'm not a pro
Retired Defense Industry Engineer; Schluter 2.5-day Workshop Completed 2013