He meant Roman Gaul.
I myself have often wondered about this subject. Over time there have been several mass migrations from the steppes pouring into Europe.
Scythians, Sarmatians, Iazyges, Roxalani, Alans, Goths, Huns, Avars, Bulgars, Magyars, Cumans, Kipchaks, Gokturks, Khazars...