s it economical to buy new after arriving in Gdansk?
The shopping malls round Gdansk are pretty good. A lot of stores you'll recognise, plus some specific to Europe that you should definitely visit.
I wouldn't bother paying extra to bring heavy clothes in your baggage; clothes aren't expensive in Poland, and due to the weather they have a great selection of winter clothes.
Massimo Dutti is a good chain (part of the Inditex Group) for decent quality clothes - they're opening a small number of stores in the American market at the moment but have no plans to go countrywide there due to logistics and brand dilution. In Poland, every large city has one now. There's also Tatuum (from the Baltics, I think) which has shops in Polish malls. Some very good stuff. Peek & Cloppenberg have all the big brands, Armani, Hugo Boss, Versace, Barbour). Most malls have a Timberland shop now as well.
For cheaper shopping, some people like Reserved. There's also H&M, C&A and all the usual suspects.