> <atque ~ ac> means more or less "as well as", "and also", "together with"
or "and >moreover".
Ac and et are not quite identical in Latin. Ac joins things or ideas which
are felt to share some kind of connection, whereas et does not have this
connotation. So if we have Gothi ac Gepidae, then at least in the best
Classical Latin, this would mean the author felt the two tribes were similar
or in some way "obviously" belonging together. But I suspect the author was
not writing Classical Latin...
Peter