Hi Monkey,
There is no easy way to tell him but by the sound of it he is going to find out sooner or later anyway so better you come clean.
What does make it a little easier is not to just present the problem to him, show him what you are going to DO about it at the same time. It is an old saying here in UK – actions speak louder than words. It is the actions that you take to help you stop gambling that are the same actions that might help rebuild the trust etc. Actions like getting excluded frim where ever it is that you usually gamble, actions like being accountable for money and time, actions like getting to Gamblers Anonymous meetings, posting here, finding more positive ways to fill time etc.
As Vera said it is also important to come completely clean. If we “drip feed” the bad news then partners etc dont know when we have finished the drip feeding, dont know when they can start rebuilding trust etc. Also of course it will help if your partner helos you with the financial barriers – trying to keep hidden debts secret will often send people back to gambling as they try and keep the secrecy.
Show them this site, there is support for Family Members here as well.
Keep posting, good luck in recovery.