South Sweden was Germanic throughout the Iron Age. Then, AD 450-510, it got invaded - from central European ancestry
. The main fort - "Borg" - involved here was Sandby, which after a massacre was left to ruin.
In order to get there, a central European would have to have pushed through what's now Pomerania. Some today argue that a language was there once, separating Prussia from whatever was spoken in Germany at that time - which, duh, was German. I don't know if that language was still around however.
Another thought - and if I may say, a better thought - is that here we are dealing with ex-Huns. Etzel was murdered at the beginning of this timeframe. Better known to Latins as "Attila", his subjects were not Huns (last I heard the Huns were being claimed for the Yenisey). Attila's fighting class were Goths; their peasantry at this time Latinised Dacians and Pannonians. Germans call them Wallacians; they now call themselves Romanians.
Anyway the Gepids et al. weren't in wonderful position to do more invasions south or west. Other Germans were already there and didn't want the Huns' "traitors". The Roman Empire in Constantinople, now laser-focused on the Balkans, was not a nut as could be cracked. From the East whence the Huns, the Avars were coming, uniting the Slavs.
Up north however - what was there? Maybe not the riches of Byzantium; but the Baltic could be fished and amber could be sold to the far side of Denmark. Recall that the Franks were building a viable kingdom, not as grand as Rome, but not too much worse than the Gaulish province had been under, oh, its Constantine III.
Against this "return" the Danes, for their own part, put up a fence, the Danevirke.
At any rate, having taken south Sweden, they'll enter Anglosaxon legend as "Geats". Some genetics hint at that too. Note Peter Heather's name on this one.