If you're tired of every new FPS game being a quasi-realistic military shooter with the soul of the single player stripped out for the sake of forgettable multiplayer, this is the game for you. No regenerating health, no on-rails segments, no faceless soldiers opening invincible doors for you. This game plays in a truly unique way, despite the aged graphics and sound effects.
You play as a WW1 veteran, called out to an island to help a friend. With your knowledge of the occult and your skill with firearms, you use both magic and technology to battle supernatural and human foes.
This game has what many today lack: variety. You'll travel through dimensions, through time, outside across grassy plains, and deep into the depths of booby-trapped caves. Enemies range from the undead, werewolves, aliens, witches, and more that I will not spoil.
It's an unforgettable experience to me. It's scary, fun, and due to minor RPG elements fairly replayable. If you can look past the dated graphics (which I'll assume you can: you're on Good Old Games after all!) you'll find a gem in this game.