He was a good Christian leader if only for the fact that he took unprecedented measures to prevent his men from raping and pillaging like all the other armies were purely because of his Christian faith. Granted, he didn't succeed completely in stopping them, but no army has been able to do that, even modern ones with all of their advanced technology and psychology.
>I don't think Gustav ever fought the Poles
Gustav did fight the Poles, though. It was one of his three major wars before getting involved with the Thirty-Years War. The others being a war with Denmark and a war with Russia.