chatGPT is a bit like google or stackoverflow etc, but maybe more directed. Don't use it's code as it's nonsensical mostly, but it can definitely summarize well what directions to take.
Other then that, if it's math-heavy/complex, maybe start with parts. try to identify what you want to be libraries and core logic etc.
you could start by making a math library in C, maybe with some bindings to a higher lvl language you want to write the GUI code in. It will be quite a project in itself already, and once done, you have part of your goal completed.
Selecting, testing and experimenting with GUI code is also tricky. There's tons of different libraries out there. You can use existing, or even write openGL/Vulkan code and write your own. If cross-platformness is important, maybe look at things like .net maui etc. as it has that feature as a core feature.
Also, before writing anything, look into how to structure such large projects. Structuring your project will help in many ways, including deciding how to break up the project, where to start potentially etc. etc.
There's many ways that lead to rome, but most will be 'chunk it up untill the size of steps is so small you feel confident you can take a step, then take it'.
