I realize that this question is almost two months old, but I'll answer it anyway because I know the problem very well. Hopefully someone will find my answer useful.
Simon the Sorcerer and many of the other adventure games at GOG use ScummVM - this is an application that makes old adventure games compatible with modern operating systems. The problem is that these games run at low resolutions. The graphics tablet built into your screen gets confused because it is only configured for the screen's native resolution. (Mine is 1280x800, yours may be different)
My solution is to run the game in a window instead of in fullscreen - that way the tablet pen behaves normally. Because the game runs in a low resolution mode, this will make the window a bit too small. To solve this I use ScummVM's 3x scaling mode. This increases the window size to 960x720, which covers most of my 1280x800 screen and as a result the game is very comfortable to play. If you have a different screen resolution, a different scaling mode may work better for you. (2x is the default setting if I'm not mistaken)
What you need to do is to browse to the game directory and open simon.ini in a text editor. Find the line:
fullscreen=true
and change it to:
fullscreen=false
Then add the following line:
gfx_mode=3x
I've played several games with ScummVM on my tablet using this solution, and it works great. Good luck!