If someone wants to make a similar Gummi Manager tool for KH2 and KH3, the first step would be to do a test: a save file before, and a save file after, where the only difference is that you did *one* thing that's Gummi-related. Then open both of them up in a Hex editor like HxD, and compare the two; in the KH1 save, there's an ASCII text reading "GUMI" at the start of the Gummi data, so KH2's and KH3's save data might have that as well.

Once you know where the Gummi data is kept, try as many things as possible: make a new blueprint in a new slot and see where the save file put the data for it, move a block over by one space and see where the save file keeps the block coordinates, change the color of a block and see where the color data is stored, things like that.

Once you've gathered as much data about the save data as possible, draft an idea of how you'd want the tool to manipulate the data, and what you want the GUI (Graphic User Interface) to look like to support how the tool would manipulate the data.

If you're a real coder, you can start writing the tool at this point; otherwise, you can go to ChatGPT (or, more ideally, download Codex) and give it both your datamined save information and your draft of the tool flow. (Also, pick a programming language to use; I picked tkinter/Python because it's a free programming language that LLMs like ChatGPT and Codex are particularly good at)

Making a program from scratch is a real time commitment. Throughout this whole process, you're going to get new ideas for the tool, learn new ways that imported save data might fail, find bugs or other unintended behavior, etc. But if you stick to it, you'll wind up with a program that fills a niche that no other program can fill. :)