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,etc for just a small amount of money and they try to gambling it. and it's depend on what gambling their have because There are also many different type of gambling.
Well the topic really stands for religious matter thats why OP asked if this is Dirty or not
But thats true that as long as the money is from good source then theres nothing to think about being dirty
It's all up to you when you hate or do you love and addicted gambling. The people who really Love's and addicted gambling is...they don't mind money where it's came from. But from the people who really hate's gambling for sure they will ask first where the money came from and for sure the first reason that they will put on their mind is the money is from a dirty place where the gamblers are there.
But thats a specific answer while we are talking about religion connected here .
But still dirty money is from bad earning but if this isnt legal then its not dirty
Religion and legal concerns are not the same and there are a lot of things which is legal but in religions its not acceptable maybe gambling as well. So if your thinking is to call an illegal things halal its not good. I know the money never becomes dirty but the way we use to earn it must be dirty avoid having bad name in gambling.