Is such thing possible? It would be that much. I mean, gambling sites are having promotions of bonuses, rewards, discounts are free spins, but maybe, as this much, it would be impossible. 1-5 BTC as winnings, would be huge already, and surely, gambling site owners won't allow players to win such amount without investment or deposit, because it would be a loss to them. I've heard of gambling sites which allows those players who cannot afford making a deposit, to have numerous 'trials' , not sure if free spins, but still, the outcome would be depending on one's luck and winnings won't surely as big as those who are making deposits, because it would surely appear to be unfair. Much better if you would try making a deposit if you are aiming for a huge profit from gambling.
Also, due to welcome bonuses, in which most of the gambling sites are offering to the new ones, your expenses would be lessen a bit. I, do personally recommend gambling sites who are having such promotion because it is practically helpful. Well, you won't have trouble finding gambling sites of a kind because most of the gambling sites nowadays are offering such promotion. I hope you would enjoy your gambling experience more, buddy. Play safe!