Archived - I would recommend using the inbuilt GUI functions, or the ImGUI library for unsafe mods. Feel free to copy some of the layouting code and logic for use with the inbuilt API, a lot of the logic is useful for using the standard gui functions.
A GUI framework for Noita, with properties and behaviors somewhat similar to HTML and CSS, and with rendering like the CSS box model. GUSGUI was inspired by EZGUI, another similar GUI library that was not finished.
Download the latest release, and extract it into your mod folder. Updating must be done manually by downloading the newest release.
local gusgui = dofile_once("mods/YOUR-MOD-ID/PATH-TO-GUSGUI/Gui.lua").gusgui("PATH-TO-GUSGUI")
local Gui = gusgui.Create()
Documentation is on the Github wiki.