For RPG Maker games it's actually relatively "simple". You really only need to know the basics of how RPG Maker works, because you do everything using the same editor the game is created with (so you'll need a copy of that version of RPG Maker).
There's going to be some differences depending on which version of RPG Maker the game was made with (these days mostly MV and some VX Ace), but the stuff relevant for something like this mod is basically the same, ignoring minor changes in how everything looks.
A tutorial
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goes over pretty much everything you need: how to create/edit maps, creating events, and using the database, which stores common events that can be called from multiple places.After that you can pretty much make whatever changes you want. So in the case of a gallery mod, it's basically the following workflow:
* Find character that initiates the scene you look for on a map.
* Find the functions that are called to show the images, text, etc. (either directly on the character, or in a common event that is called from the character).
* Copy the relevant parts to a gallery map you created.
* Make cosmetic/functional changes as desired (remove area transitions and anything that modifies the game variables; replace state checks (lust, current party members, first time scene) with choices so the player can choose the variant they want to see; ensure all animations play correctly).
