There is a short explanation of how it works, and which display panels are supported.
Note that using the same approach on panels that are not four grey levels "capable" may look horrible on those; the grey parts look "dirty".
Sorry to bother you but I have a issue I haven't been able to track down.
I'm successfully using the "GxEPD2_4G_Example.ino" with my EDP.
However the serial output is pushing a lot of what looks like testing print on the IDE serial monitor (see below). I'm pretty sure they are for troubleshooting, however I've not been able to suppress them.