it's easy to get a full gallery by cheating and doing the true ending.
start a new game, save, then go to
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put the save file there. give yourself 99 of all keys, the fire ring (immunity to fire and lava, this will massively save you time in the 2nd to the last layer, you stop giving a fuck about lava anymore and just walk directly to the next floor, heck you can even skip/walk over fire traps with this ring on), hero's sword, witch's sword, bow, hero's armor, lots of teleport scrolls.
So what do you do? Speedrun the good ending, everything in this game is timed, take too long and all the girls will die. talk to all the girls in the start of the dungeon then take the key shortcuts to go to the boss (look for doors with colored gems), save the girls (do not rape them) teleport, talk to them in the inn, go back, and do the next layer. Beat the witch, enter the witch's house, and finish the game. Now you've unlocked the true end. Go back to the gallery and there should be an orb near the entrance that asks you if you want to unlock everything. How fast? Will not take you an hour, in-game, less than 7 days.