In my opinion to be a good gambler you must be:

1- Brave, to risk significant sums of money when you feel that victory is very likely. that way yu can earn a lot of money.
2- Mistrustful and can control his greed, so as not to waste money, and know how and when to risk them.

However, the most important thing is how to balance these two totally different characters and here comes the complexity of gambling.