I had wondered whether Gosia was short for anything, since I thought the Poles were at least as fond of using nicknames as the Russians, and that it was, therefore, likely to be a nickname rather than an "official" name.
A bit of Googling pointed to the full name Małgorzata, which I had never heard before, either. Searching enwp for that brought lots of hits, though, so it seems to be a fairly popular name.
Searching for it on pl.wikipedia.org finally told me more; I first looked at the interwiki links and then at the text, both of which indicate that it's the Polish form of Margaret.
(The article also listed twenty other nickname forms besides Gosia... those Poles really seem to be creative in their nicknames. Though I can imagine that it also helps to distinguish if you have, say, several Margarets in one school class.)