you can always calculate how much you can earn with gambling but keeping a record of your losses and wins and then add things up to see how much you have lost in total against how much you have won and you can easily calculate it to see your own records.
But we can not say exact amount how much we earn in gambling. Because one day we can make profit and next day we lose that amount and it will repeat so here profit is nothing only lose. I think earning in gambling means I won my bet in sport betting that's it I never made a profit in casino base game like Dice etc.