Thanks so much for playing.
If you are a complete GB Studio beginner, my first bit of advice is to try making something simpler first to get familiar with the tool. You can probably, maybe make this type of game just using the visual scripting in GB Studio, but it would take a long time!
My second bit of advice is join the GB Studio Discord server and learn to love the search feature.
This game heavily relies on GB Studio custom script events that I wrote to load level layouts, loot placement, monsters stats, and really all kinds of other stuff. I also used plugins that others wrote.
Basically this game has a scene for every room layout, a title scene, a shop scene, an inventory scene, a battle scene, a game over scene, and a win scene. There are very few actors, triggers, or colliders. Just lots of scripting and game state tracking.
If you have more specific questions, I may be able to help. But that's the high level look.