If you use the GPL code then yes you will have to open the source. So use the GPL code to build the game with and if you're ready to release it (and it's not just a free game) then buy a programmer seat for the library.

if you don't mind building on top of a very thin multi-platform layer i suggest having a look at boost.asio (note that asio has not been accepted into boost yet but they're working hard to get this done. most boosters see asio as a very important library). it might be just the right thing providing basic abstractions. also you can build a replication feature on top of asio (there is already something implemented using boost iostream). worth checking out, i'd say.