Hi,
in the .gir file, the function gthree_geometry_add_attribute and others are marked introspectable="0".
This seems to be the case for more functions ingthreegeometry.h and in other headers.

E.g. gthree_geometry_add_morph_attribute is marked introspectable, so I think to fix gthree_geometry_add_attribute, an annotation for the return value is needed.

BR
Peter