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.
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.