RIC VI only records this reverse being issued from Rome for Constantius I (RIC VI Rome 43). These would have been struck for all the members of the Tetrarchy though, so it is not surprising to find an unlisted one for Galerius.
As far as authenticity, I don't have much experience with silver issues, but from other examples I have looked at in the past, I don't see anything about this one that would immediately condemn it.