  We report on the magnetic properties of GaN:Gd layers grown by molecular beam
epitaxy (MBE). A poor reproducibility with respect to the magnetic properties
is found in these samples. Our results show strong indications that defects
with a concentration of the order of 10^19 cm^-3 might play an important role
for the magnetic properties. Positron annihilation spectroscopy does not
support the suggested connection between the ferromagnetism and the Ga vacancy
in GaN:Gd. Oxygen co-doping of GaN:Gd promotes ferromagnetism at room
temperature and points to a role of oxygen for mediating ferromagnetic
interactions in Gd doped GaN.
