If someone needs a simple solution to rotate an object - I found this:
get_node("Node Name Here").rotate(Vector3(0,1,0),0)
if you want to ADD rotation, just change the last zero. Vector3(0,1,0) represents rotation by Y axis, (1,0,0) is for X and (0,0,1) is for Z, I think.
get_node("Node Name Here").set_rotation(Vector3(0,0,0))
and this is how you can SET rotation.
It all seems to work with PI, not degrees. So instead of 90 45 30 there should be PI/2, PI/4, PI/6 etc.