Hello :)

As the title of my post says, MM7 (GOG version) refuses to launch, now that I've installed the Grayface patch. When trying to start up the game, my computer screen goes black for a moment, as though the game is about to run, and then it just reverts back to the desktop.

I really want to get the patch to work, so I can install the Amber Island mod (I know you can download the mod standalone, but that didn't work for me).

Any ideas?

The vanilla GOG versions have Windows 95 compatibility mode enabled in the executable. It is needed to run the GOG version out of the box
You need to disable it to run the game with the Greypatch enabled. The same for MM 8 Day of the Destroyer. MM 6 seems to have the compatibility mode disabled by default, so no trinkering is needed.