Gothic is actually the oldest Germanic language, in the ancient times Goths lived in the southeast of Europe, it is usually believed that they migrated from Scandinavia but I can't find any linguistic evidence for this migration, all evidences actually show that they migrated to Scandinavia, for example we see numerous Latin and Greek words in Scandinavian languages, such as dragon, wine, ship, pipe, ...
Gothic is not the oldest Germanic language. It's the oldest
fully attested Germanic language for very obvious reasons, i.e. it was spoken near to literate societies and was under very strong Roman and Hellenic influence from a very early age. Gothic was already specifically
Eastern Germanic, and not generically Germanic, let alone Proto-Germanic, so when it's first attested Proto-Germanic had already split into at least 3 different branches of Germanic dialects/languages since at least a few centuries earlier.
However, the earliest attestation of an undisputed Germanic language can be found as early as the 2nd century A.D and is identified as a very early form of Proto-Norse that is still reasonably close to Proto-Germanic. That proves that Germanic languages were already spoken in Scandinavia even before Gothic appears in the written historical record. Besides, the fact that Finnish, Estonian and Saami have a numrous adstrate of Proto-Germanic or even presumably Pre-Proto-Germanic loanwords, having borrowed forms of the Germanic words before they changed significantly in the latter branches like East Germanic, also indicates that Proto-Germanic and probably its earlier linguistic stages was spoken nearby the Finnic and Saami tribes, which were probably natives to Northeastern Europe, not to Southeastern Europe or elsewhere further south.
Goths do not represent the earliest linguistic or sociocultural stage of Proto-Germanic peoples at all. Genetically, as the latest studies have demonstrated, they also seem to have been pretty mixed with lots of Northern European, Eastern European and even Southeastern European elements, so they look more like a population formed out of a relatively recent ethnogenesis, not a relic of older times.
A major factor to confirm that Goths came from Scandinavia, apart from the relatively contemporary documents that claim just that, is that, well, the Goths appeared in Southeast Europe and dispersed from there to many parts of the Roman Empire in the 4th to 6th centuries. They were not mentioned in the Roman and Hellenic sources in the Balkans well before that, and they seem to have been a "novel population" settling in the region in the Late Antiquity. So it clearly makes more sense that they came from outside of the Roman Empire into it, not the other way around.
Also, Goths did not live in Southeast Europe. They migrated during historic times to it, and there are lots of documented evidences of that migration. Goths had originally lived in the lands roughly from Poland to Western Ukraine/Moldova, therefore in North-Northeastern Europe. They were decisively pushed away by the Hunnic invasions and possibly even earlier by other movements of people.
Wine is a Latin borrowing present in all branches of Germanic languages (not just East Germanic), so it might've come from anywhere between Gallia and Britain, in Northern Europe, and the Western Balkans, but Latin was hardly spoken by a sizeable part of the population east of Carpathians.
Dragon does not count as a general Early Germanic loanword from Greek. It's held to be a much later borrowing from Greek and only appears in Middle English (medieval era), for instance. Germanic peoples in Northern Europe were not isolated from the Mediterranean world, by the way. There are lots of archaeological evidences of widespread trade and cultural contact between them since the early Roman Era or even earlier. As with
dragon, a large part of the Latin and Greek loanwords in Scandinavian languages seem to date from the Middle Ages as they had even closer contacts with the Mediterranean cultures, but it started even earlier, because Scandinavians and other North Europeans were profoundly influenced by the Roman Empire even before their Migration Period.
Why do you say
ship is a Latin or Greek borrowing? It's also present in all Germanic languages and can be reasonably linked to the PIE root
*skey-,
-skeyb, so I see no reason to believe it
should be a Southeast European borrowing.