With the amount of money I have spent whiles gambling and haven't won any major jackpot, I am not going to be donating anything to any charitable cause unless I have made a huge profits and to my mind most people view money from gambling as dirty so I doubt any charity will be thrilled to have them.
Well I think its not necessary to open up where your donation came from, what important is you help this charity. I never had huge donation in charity as I dont win huge as well but if you really want to help others there's a will to do it. We dont need huge amount, we can share what we can afford.