I have never kept it a secret. At the beginning it is normal for them to be suspicious, but when they see that you regularly make profits, they believe what you have told them. The problem is the gambler loser, who from time to time loses more than he should. Those many times en up with their partner breaking up.
One thing many people lack especially people that are married is understanding. Study your partner and know the things that they can tolerate, if you observe that your partner is the type that's mature about things like this then there's no need to keep bottling it up and trying to hide it. Most people keep gambling as a secret from their spouse because they know that they are addicted to it, they are trying to cover their habit