Hmm… difficult to qualify what a hidden gem is in this day and age because any game without mainstream appeal can be a hidden gem if you’re not into its genre.

Any shmup fan must have heard of Hellsinker, for example, at some point, but you probably haven’t if you’re not into shmup.

My favorite hidden gem though is The Red Star on PS2. Seamless transition between twin-stick shooting and belt-scrolling brawler, and almost equally entertaining in both modes. You unlock skills and get ranks at the end of every mission, which’s very Japanese even though it’s developed and published by a western studio.