Depends on the gun and the gas, but it doesn't perform as well as high pressure air or CO2 in the cold. This is really something you should search forums for though as 'cooldown' (the effect of rapidly cooling magazines as the gas within them escapes) between different guns as well as their contraction in cool weathers is well documented. In short though, cold weather can play hell with any gas blow back gun that runs on green gas (i.e. propane with some additives that you see sold at many sites, used by players to charge their pistol magazines etc.). Many guns are better at dealing with this than others, but none are perfect and that's why we're seeing a major move over to high pressure air (HPA) in the UK right now, because it works all year round (though the initial cost is quite high).
Don't get a GBBR as your first gun. You absolutely will regret the costs that it brings and the knowledge that you'll need to fix things when the go wrong. An AEG works well in all environments above sub-zero and is a good fall-back to have as a beginner (as well as generally being cheaper than GBBRs). I'd always recommend a well-branded M4, AK, MP5 etc. AEG as a gun to start with and then build from there. That way if something goes wrong then you can still play.
