I checked out "Countdown"! Thanks for the heads-up; I suspect it can be paired with a "Coming Soon" configuration?!
Meanwhile I might be using the wrong terminology, but to me "gateway" is the page (typically smaller than usual in size) that one encounters when first accessing a web address where further admittance is a matter of consent. Is it called something else? A "door" maybe?
I found one WP plugin" "Content Warning." Unfortunately its almost a bit too simple, and doesnt seem (easily) customizable.