This is a library built for GJS to allow defining GTK widgets in a declarative way. It also provides services and other utilites to interact with the system so that these widgets can have functionality. GJS is a JavaScript runtime built on Firefox's SpiderMonkey JavaScript engine and the GNOME platform libraries, the same runtine GNOME Shell runs on.
It was heavily inspired by EWW.
Currently only Wayland is supported, but there are plans for X11.
To get started read the wiki.