Unfortunately I haven't seen anyone else reporting the game minimizing when clicking on the gamma slider, so I can't recommend a workaround for that at this time (if something you try does end up working, please post it so if anyone else has the same problem they'll know how to handle it).
The only recommendation I can offer at this time would be to delete your settings file which can be found here:
C:\Users\YourUserName\AppData\Local\ShopSim\Saved\SaveGames
I hope this helps!