It depends on Money that they use for gambling. if the money came from a illegal job etc then that's a dirty money because of addiction on gambling some of people doing illegal just like rubbery or etc for just a small amount of money and they try to gambling it.this kind people are those who have a small source of income, and it's depend on what gambling their have and Status of Life they have.
In my opinion, dirty money can be called only what is obtained by deceiving other people. Many Goverments themselves trade in arms and administer gambling, while prohibiting their citizens from doing the same. That is why I believe that only cheating other people can be an unworthy way to make money. And if the casino does not have special settings to commit fraud, then his money can not be dirty.