@export_custom(PROPERTY_HINT_RANGE, "-360,360,0.1,or_greater,or_less,radians") var rotation : Vector3;

This would be the most recent method of doing this, I am unsure when this was added but it works in 4.4. Adding this answer incase anyone is still looking for good ways to replicate the rotation transform.

It does automagically change the values from degrees to radians under the hood just the same as the transform settings for nodes do.