Pretty much, when talking about big gains, you must consider the type of people you are talking to. There is indeed a need to be sensitive about the things that come out with your mouth, especially when it won't do you any good. But, if talking about big gains would bring positive impact to others, such as, it would give motivation for others to move forward, try themselves and improve, you might consider not to hold back.
When it comes to gambling games, it is no need to say anything to other people, especially about winning, but you can tell them about your losses. You can say that it is hard to win in one gambling game, but the truth is you win that games.
I don't think that talking about big gains would bring a positive impact to others because we can make those people chasing the win money because they want to get the win like us. So that can make them spend too much money. Perhaps, we can share our winning with other people who can control themselves while they learn something from us.