Gunman Chronicles by Rewolf Software is a commercially released product based on the GoldSource (Half-life) engine. Starting off as a mod that was scooped up by Sierra during the gaming hype around half-life, Gunman Chronicles was granted a retail release and lived a short retail lifespan.
Unfortunately shortly after release, Rewolf Software was disbanded and Sierra was in the process of being consumed by Vivendi. This meant that the publishing rights were lost to time during these events although they are now rumoured to be with Valve. IIRC, to prevent Gunman Chronicles disappearing completely, some of the original design team banded together to form a new studio, reverted Gunman Chronicles back to it's mod form and released it back into the world where it has since been preserved on ModDB.
If GOG however could get it re-released here, a new audience could discover what is possibly the best GoldSource campaign to ever be released at retail. A wild adventure containing Aliens, Dinosaurs, Robots, Robo-Dinosaurs, Space Cowboys and an insane AI who could rival both Shodan and GlaDOS. Customisable weapons with multiple weapon modes, driving mechanics and a flair for dramatic set-pieces all appear within it's galaxy-hopping campaign.
I played the demo version through multiple times back in the day because it was fun and unique, containing content and levels that don't appear in the full game as you are hunted across a jungle planet by hungry dinosaurs and space cowboys. When I obtained the full version via a budget re-release just before the publishing rights collapsed, the campaign turned out to be even more wild, with superb set pieces, interesting story and clever level design, an enthralling ride from start to finish.
Gunman Chronicles is one of my most treasured games and if it was ever re-released on any digital storefront be it GOG or Steam again, I suspect many people who take the plunge into it's campaign would absolutely feel the same.