It's not gambling that can build your character but it's important that you have a good and strong character if you are going to gamble, you cannot justify that you are gambling because you want to develop a good trait, it's like justifying why you are spending and losing a lot of money, I prefer to look at gambling as it is and that is to get entertain, and spending more money and time will lead you to lose money and time.
Are you going to recommend gambling to build character it's like heading them into a quicksand.
I agree in order to develop good characters of out of gambling first we need to be capable of learning out of anything. Addicted gamblers will never mind about learning anything while they are gambling. Such type of gamblers will keep on gambling regardless of whatever outcome they are facing so when they are not focusing anything outside from gambling then you cannot expect them to be developing good characteristics from their activities.