Thanks @mransome, this worked perfectly for me. Now happily Tormented.
mransome: I believe this has been fixed with the latest version of Wineskin.
As a temporary fix, until GOG release an update, you can try the following:
Navigate to your Applications folder and right-click on the "Planescape Torment" application then select "Show package contents". In the window that appears, navigate to the following folder: Contents>Resources>game. You should see a "Planescape Torment" item here. Again, right-click this icon and select "Show package contents". In the window that appears double-click the "Wineskin" icon.
This will open the Wineskin interface. Click 'Advanced' and then click the 'Set screen options' button. In the 'Other options' panel make sure 'Use Mac driver instead of X11' is checked. Click 'done' and then close the Wineskin window.
There may be unexpected quirks to using the Mac driver, however the game appears to function correctly here.