spend more so earn more

You can't just spend on advertising without studying or making an effective plan, it will just a waste of money. I think you can visit other gambling site list down their promotions and improve it to have an effective promotion and advertise it in the forum to get players. Giveaways or signature campaign that pays in the account of the users in the gambling site could be a good idea too.
One of the things with marketing is that even if you spend a lot of money your marketing campaign could end up being a failure as you are targeting the wrong crowd and they are not interested in the service that you are offering.
As such the money needs to be spent while making sure that it reaches the people that may be interested in gambling in a new casino, but this is something difficult to do as convincing a player to move from one casino to another is always difficult, but if to this we add that established casinos are also trying to lure new clients to gamble with them then it becomes way harder for a new casino to succeed unless they happen to have something that makes them unique and different from all the competitors they have to face in this market.
As a business then it would be sensible that you would really be that going after on a place which you do need a specific traffic which those money allocated or spent would be at least worth of but if your budget
permits you then going into other mediums wouldn't be a bad idea at all because you could still get potential players/users if those ads would really get or caught their attention.
Every business does need exposure which its really just pretty basic that you would be allocating budget for that.