You may have experienced two issues installing and playing the standalone GOG (non-Galaxy) versions of SCUMM games: 1) The installer gives you a permission error; 2) the SCUMM windows starts mysteriously shrinking while playing. Both can be fixed fairly easily, assuming you already have the latest version of ScummVM installed. The steps, using Sam & Max Hit The Road...
Make sure you download the standalone installation package...
After download, when you try to open the installation package, you may get this message...
In which