Let us be honest first: a gender reveal is not a separate event type here. The closest fully supported setup is the baby shower, and that is exactly what you build a reveal on โ the same page, the same reply flow, the same schedule and directions. What it gives you that a printed card cannot is control over who sees what, and a way to keep the secret to the people you chose.
The reveal hinges on two things. The secret has to survive until the moment you decide, and the guests have to follow the colour instruction correctly. A hosted, password-protected page handles both: the details stay behind a code you share deliberately, and the theme-colour note is spelled out in one place everyone reads before they arrive.