Download and install the GOG galaxy client to access a free install of System Shock 2
Download is about 135MB and updates with almost as much again on first launch.
Game is a classic, regularly $9.99. To claim it, you need to get to this ‘stress-test giveaway’ page via the galaxy app. I searched the store for System Shock 2, and that page has a link above the price & shopping cart back to the giveaway page. Via the Galaxy client, that shows a simple “Claim” button, and then it’s in my GOG library.