The problem doesn't have anything to do with P3D, the Hotfix or GSX, and it has been discussed many times on the forum, it's caused by the fact the airplane you are using (Majestic Q400, is unique and use a non-standard flight model, so GSX cannot possibly know if its engines are on or off.
As suggested in all other threads about the Q400, using the default key that shuts off fuel pumps (by default it's CTRL+Shift+F1), should work.