I too have had the hissing problem as described on the Toto Drake toilets. I installed two Toto identical drake toilets with the Gmax flush valve. Both toilets would hiss constantly after a flush. It is intermittent problem and it started on both toilets at the same time. However, I believe I have found the problem. I have not seen the fix posted anywhere else, so I will post it here.
On the bottom side of the float arm, there is a small rubber pad. This pad comes into contact with the fill valve assembly cap and closes the valve when the tank is full. Over time, the small point on the cap makes an indentation in this pad. As the indentation becomes deeper, I believe that the float arm fails to seal against the assembly cap properly. The attached photo shows the pad.
The pad pops out of the arm easily. I removed the pad, turned it over, and reinstalled it. This seems to have fixed the problem on both toilets.
If you have problems with the Gmax flush valve running, I suggest trying this solution before buying anything. While you have the float off, remove the assembly cap and check for debris under the rubber gasket as this could cause issues as well. I cleaned the filter, assembly cap, and gasket as described in other posts, and that alone did not work. Flipping the rubber pad over did the trick. However, this is only a temporary solution as the indentation will reappear on the other side of the pad and the problem will come back. I will be contacting Toto regarding that.