Yes, you need to gamble it all. If you have any luck at all, you should be able to get around half of it back. If you get on a lucky streak then you should get all of it or more back. If they gave it to you all at once in cash, they would give you a 1040-G or 1099 for the full amount, but since you are getting casino play, you will only receive a 1040/1099 if you win over $1,200 at any one time (on the slots).