GLARBS is heavily influenced by Suckless, the source code is the documentation. Check out the programs config.h.
There is A friendly guide to GLARBS for the whole system, including the binds.
Press Super+F1 to view it. Also there are man pages for dwm, dmenu and st.
This website has more in-depth information, tutorials and guides.
For support you can file an issue or merge request in the corresponding repo on GitLab.
This key can usually be found on the bottom-left of your keyboard, next to the Alt key, and usually has a Windows logo on it. It is sometimes called the Windows key or system key.
If you have an Apple keyboard, you will have a Command key (cmd) instead of the Windows key, while Chromebooks have a magnifying glass instead.
File an issue or merge request in the corresponding repo on GitLab. If the code is universal, independent of the operating system, you can instead file an issue or merge request in the corresponding upstream repo.