More than 50% of the world population gamble in one sport or the other and some are making good money while some are still hoping and praying for their luck to shine.
No I won't consider gambling as a source of income. We can say something is a source of income when you are sure that the income you intend getting from there is guaranteed, and you don't know the intervals that such money will come, if it will ever happen. I haven't seen where anyone said their source of income is gambling. Or where someone said his/her occupation is gambling, so how can it be a source of income? Gambling is just a trial something, if you are lucky you get something from it periodically, and most times nothing.