GTK
@ https://www.gtk.org/
An open source cross-platform widget toolkit, popular with the Wayland and X window systems (most common on Linux).
Qt
@ https://www.qt.io/
Popular cross-platform software for creating GUIs.
Flutter
@ https://flutter.dev/
UI development software created by Google that can output software for many different platforms, including web browsers, with a single codebase, using Dart.
Electron
@ https://www.electronjs.org/
A nodejs-backed SDK for building standalone cross-platform desktop applications using a minimal chrome web browser as the GUI renderer, so existing web development tools and languages can be used for native-like apps.
Tk
@ https://en.wikipedia.org/wiki/Tk_(software)
An older graphic widget toolkit that has cross-platform support. Often used with the cross-platform scripting language Tcl, it also has bindings to other languages.
Nuklear
@ https://github.com/Immediate-Mode-UI/Nuklear
Cross-platform GUI library, uses immediate-mode design pattern.
GUI Under Linux
@ https://www.baeldung.com/linux/gui
Overview of the graphics system of Linux to build GUIs.