By having a simple system to allow a user to enter the gambling zone, like a command block teleporting them, you can then use an RS-NOR latch to remember if a player is inside the gambling zone or not. You can then use an AND gate connected to the RS-NOR and to a button, which then connects to the command block to teleport the user into the gambling zone. It would also change the RS-NOR latch to the on state. Inside the gambling zone, you can have a button to leave, which would connect to a command block to teleport the player, and also to the reset line of the RS-NOR.
Here is a (hopefully) simple diagram demonstrating what you can do. The two command blocks (Com. Block Out & Com. Block In) would contain the teleport command (
/tp @p x y z) to go out of the gambling zone, and into the gambling zone, respectively.
This circuit will make sure that only one person can go in at the one time, and they can also not allow other people to enter.
As pointed out in the comments, this system will not be able to check for if a player leaves while they are inside the gambling zone. However, by adding the simple circuit in the diagram below, possibly to a location underneath the gambling zone (so you can use the
r argument to check for players), you can fix this issue. Basically the below circuit will find if there are no players inside the gambling zone, and if so, will output a signal that connects to the RS-NOR latch on the above diagram...
The command block
Com. Block Check would contain a testfor command (
/testfor @a[r=5] for example).