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.
Overall, the positive evolution of gambling is possible only for the gambler who are all open to learning. If you ignore everything then there will be no room for any of positive things.
Which is why even if I think the OP is not wrong and you can learn some important life lesson out of gambling, I think the majority of gamblers do not really learn anything substantial out of it, and this is because most people are just interested in having some fun and nothing more when they gamble, so even if a life-changing lesson could be learned from what they are experiencing it is difficult they will do so as that is not their intent.