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Not sure what you mean with GUI, but you can use it with Jupyter notebooks.

More like writing/clicking math (like in MS Word or some LaTeX editors) that is then converted to SymPy instead of writing stuff in Python directly.

It should be relatively straightforward to build something with Visualequation, latex2sympy, SymPy, and maybe IPython's QTConsole.

Edit: That might be overcomplicating things, there are already better alternatives: https://tex.stackexchange.com/questions/57068/wysiwyg-latex-...