After I collected the hardest triangle third and the highest triangle last, the gate hadn't opened because it didn't register the hardest triangle. I decided to check up on the chest in the dark room again, and sure enough, it was closed again, even though I knew for sure I had collected it. I went back to the last gate and this time, all but the highest triangle were there. I had to go back for that one as well before I could finally complete the game. Needless to say, I cried.
 "The Valley Rule"){kind=icon}
 "Super Villainy"){kind=icon}
 "Socrates Jones: Pro Philosopher"){kind=icon}
 "ELEPHANT RAVE"){kind=icon}
 "Flaming Zombooka 2"){kind=icon}
 "Free Rider 2"){kind=icon}
;)