Godot Doc (try saying that 3 times fast) advises that you avoid using Euler Angles and for good reasons too. in fact i think that guy's method was pretty good for the century he was in but not by today's (You'll never be able to fly a plane accurately using them).
This probably directly answers your question
Spatial.set_rotation_degrees(Spatial.get_rotation_degrees() + Vector3.FORWARD)
But learn the Godot approved way first and also have a look at the Basis and Quat sections