The aspect of gambling is not only about the person, but also in all the people that lost theirs giving it to you. In the same like supposed the money that was put in the envelope came from selling drugs (and you are aware of it), would you still use it?
That's a tough call. I know wrong is wrong but, to me, drugs are more wrong. If I have earned my money honestly, am I accountable for how the person who gives it to me has earned it? That's what I would think, but my heart I would know it isn't right to accept drug money. But I also couldn't say to the person, "I can't accept your money because it's drug money. Find money from another source to pay me back." In reality I would probably forgive the debt and tell the person not to pay me back. I would write it off as a "that's what I get for lending money lesson." Maybe the dividends would pay off in the long run.