I usually need to do a specific from GlyphsApp import OFFCURVE, LINE etc., not sure why your first line does not work. What happens after you add it to a path?
--- a/ObjectWrapper/GlyphsApp/__init__.py
+++ b/ObjectWrapper/GlyphsApp/__init__.py
@@ -5473,7 +5473,7 @@ Implementation of the node object.
For details on how to access them, please see :class:`GSPath`.nodes
-.. class:: GSNode([pt, type])
+.. class:: GSNode([pt, type = type])
:param pt: The position of the node.
:param type: The type of the node, LINE, CURVE or OFFCURVE