GOG are offering the Microsoft Windows PC version of Unreal Gold for free until May 24, 2018, at 5:00 PM UTC.
To claim your free copy, simply visit GOG's homepage and click on the "GET FOR FREE" Unreal Gold link on the top (assuming you're already logged-in to your account).
The list price for that game is $9.99. It features over 47 single player missions, and 20 multiplayer levels.
Exclusive to the DRM-free GOG version are special graphical and performance enhancements for modern graphics cards. It's compatible with the Windows XP, Vista, 7, 8 and 10 operating systems.
Image credit: GOG.com