The freebie is Unreal Gold, to be specific, which bundles the original game with the Return to Na Pali mission pack. That's 47 "eye-popping" missions in all, plus 20 multiplayer levels, 13 "guaranteed giblet" weapons, and all the graphical enhancement for the latest 3D technology, including Direct3D and Glide.
I kid, but Unreal was seriously cutting-edge stuff in 1998. The actual gameplay was perhaps a little less-than-inspired, but it looked so damn good that nobody cared. More significantly it launched an engine that remains relevant to this day, and some tournaments that are still around too.
The Unreal Gold freebie will be up until 10 am PT/1 pm ET on May 24. The rest of the Unreal games on GOG—Unreal 2: The Awakening SE, UT GOTY, and UT 2004 Editor's Choice Edition—are on sale for 80 percent off until May 29.