Thank you for this submission! Always happy to see people using Godot :)
As for the game, I did not get the rules from the start, but when I got the hang of it (and when I understood that useful units also double as simple deletable figures) then it got easier and more enjoyable.
My two complaints are that I would love to be able to swap gems both vertically and horizontally, and also that for now winning highly depends on initial setup. One time I got 4 attackers at the top from the very beginning, and won just by doing nothing.