I'm glad you're enjoying the game! :)
Unfortunately, yes, the Gallery Cheat unlocks ALL the scenes in the Gallery regardless of whether you've met the characters or not. Ren'py has a weird problem with remembering when you've seen certain events or not, so the whole thing can be a little buggy. The thumbnails aren't TOO spoilery, though, so I hope it didn't ruin any surprises for you!