I'm impressed!
You really seem to have understood how to use ZGE even though there
is so little documentation, good work. That makes me happy!
About deltatime: Since you only set an initial velocity in OnSpawn, you
do not need to use deltatime. If you modify Velocity or Position in an
expression in OnUpdate, then you need deltatime.
I would make the rocks fall slightly slower though, they are too fast
for my reactions
