The leak problem is a guest-list problem
A gender reveal survives or dies on one thing: how widely the answer is known. It leaks less from a slipped word than from an invitation that got forwarded, a reply thread a curious cousin read over someone's shoulder, and a group chat that grew past the people who were actually meant to be in it.
So the real work is the list, not the confetti. This is a baby-shower page used for a reveal, the closest supported type, and the trick is to make the invitation itself hard to spread. Give the page to the in-the-know only, gather replies there rather than in the open chat, and keep everything a draft until you are ready to release it.