The Goth are not a single sentient race, but a classification of what the Enlar and Gallite have called semi-sentient.  When the Union was formed at the start of the Ancient Ignus War, all space fairing races where asked to join the Union and fight to contain the Ignus threat.  At that time there were many races in the Galaxy that had not yet discovered a means of inter-stellar travel.  Those races were not invited to join the Union after they had progressed technologically to the point of interstellar travel.  These races became the focus of oppression by the Union races, and in lieu of a true threat, the Union focused it's attention on preventing the Goth from becoming a threat.  The doctrine of "Preventative Action" was first proposed by the Gallite, but was soon adopted by all Union races as a means of preventing the Goth from becoming the "next great threat to the Union"