There are evils that are always associated with gambling. One does not obtain the wealth through work or gift.One obtains wealth at the expense of others parting with theirs. If it is conclusive that smoking causes cancer, then smoking is sinful because we are commanded to everything to preserve our physical body. If dancing is used to entice someone into adultery or to think adulterous thoughts, dancing is sinful. If drinking leads to drunkeness, it creates a rish to others and to oneself. If you believe that you can do any of these things without falling into those sins, fine. However, I don't think that most people are that strong.
By the way, it is unusual that this guy wants to tithe his winnings since the Church of God, to which he is a member, has consistently taught the evils of gambling.