Where I have to fix this Mistake?
There's some kind of strange bug in P3D, which results in the menu size becoming smaller when loading certain 3rd party airplanes. We don't know why it happens, but it's of course totally unrelated to GSX, since what you are looking at, is not the "GSX menu", but it's the standard Simconnect menu, used by all 3rd party applications with a menu, and we don't have any control over its appearance or its size.
The menu can be resized with the moust, but some users don't realize this, so they think some entires are missing, but it's just the menu that became too small to show them all.