Not sure about Unity, bot Godot is already used to build tools, like Pixelorama (pixel art graphics editor, a bit akin to Asesprite), RPG In A Box (game engine targeted for RPG games), Bitmapflow (tool to generate in-between animation frames), and probably more I don't know about.

Well, if I remember correctly, the Godot editor is written in Godot.

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Supermancho 4 hours ago | parent [-]

Godot is written in C++ It may have some GDScript in there, but I don't think so. The sourcecode is available: https://github.com/godotengine/godot

	▲	hofrogs 2 hours ago \| parent [-]
		The C++ code there (at least in the editor directory) initializes and configures godot ui components that the editor is made of