Goths originated probably in Pomerania but later occupied most of Eastern Poland, both North-Eastern and South-Eastern. Until the 1st century BC or the 1st century CE, parts of Western and South-Western Poland were occupied by the Lugians (probably Celtic-speaking people), later by the Vandals.
I don't think that all of them abandoned those lands. Evidence (both archaeological and genetic, as well as written sources) suggests that Slavs came and mixed with those previous inhabitants.
But there is no consensus as to where was the Urheimat of the Proto-Slavs located.
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Currently there are several research projects on the Migration Period in Poland:
https://www.ncn.gov.pl/finansowanie-...cz?language=en
http://www.mpov.uw.edu.pl/en/

