A total conversion of Half-Life gone retail.
Gunman Chronicles is basically one of the various fan made mods which uses the Half-Life engine and in turn it takes advantage of it's powerful scripting abilities. Although Gunman Chronicles is not as long as Half-Life and it's environments aren't as varied, it retains that same feeling of suspence the Black Mesa facilities embodied. You'll never know when the floor will give away or the ceiling will collapse and it's surprising that more FPS games haven't taken advantage of such situations.
For the most part Gunman Chronicles is a mixed bag of sorts, many of it's textures are extremely clear and detailed, most of it's sound effects are of a high quality, the enemies are well animated, there's a lot of good level design, etc. but this isn't consistant throughout the entire game.
Gunman's inconsistancy is it's biggest flaw and it really does end up feeling like a fan made game rather than a commercial one. The textures become too colorful and unrealistic, the sound effects become repetitive as do the enemies, the maps lose their charm and just seem bland, etc.
Also, Rewolf's premier title holds too many similarities to Half-Life. Being based on the same engine is one thing but using practically identical intro screen, intro sequences and SFX (and the list doesn't end here) just goes overboard for a commercial game.
The Bottom Line
Gunman Chronicles tries to introduce various new gameplay ideas but those quickly become cumbersome, instead of enhancing the game, leaving it as just another average entry in the crowded genre.