There is a A friendly guide to GLARBS for the whole system, including the binds. Press Super+F1 to view it. You can also read it as a pdf.
Man pages
There are man pages for dwm, dmenu, st and many other programs. Type man "program" in the terminal, e.g. man dwm to view it.
Suckless programs
GLARBS is heavily influenced by Suckless, the source code is the documentation. Check out the programs config.h.
Videos
The many illustrative videos on Luke’s YouTube channel about LARBS, some of which are easily watchable in GLARBS by pressing Super+F2.
Information in some of the videos must be customized for GLARBS.
Mental Outlaw’s Youtube channel about Gentoo, some of which are easily watchable in GLARBS by pressing Super+F2.
HMagellanLinux’s Youtube channel about Gentoo, some of which are easily watchable in GLARBS by pressing Super+F2.
Git repos
By reading the current and closed issues on the GLARBS and LARBS repos.
Gentoo wiki
Official Gentoo documentation as well as community-contributed content can be found here.
Through trial and error
Just install, dive in, have fun and enjoy!