The link to the free game is at the top of GoG's homepage. The game is normally $10 US, and more information is here: https://www.gog.com/game/mdk
Since it was published in 1997, the graphics look pretty aweful (I don't think it supported 3D accelerators), but no worse than any recent indie pixelart game on Steam. The gfx are much more attractive on the sequel, MDK2, but it is currently full price on GoG.
Perusing GoG's site, i also noticed that a lot of EA titles (oldies of course) are 75% off. Some I would recommend are Dragon Age Origins, Ultima 6 & 7, Wing Commander, SimCity 4, Dead Space, & Jade Empire: https://www.gog.com/games?devpub=electronic_arts&sort=popula…
The video game that first had a sniper gun with a zoom if I remember correctly.