I think, it would be a nice idea to add Keycloak to Gentoo repo. https://www.keycloak.org/ Reproducible: Always
This uses maven to build, so it's unlikely that a source package will be around any time soon.
I created this package under my own overlay (https://github.com/gentoo-mirror/ryans/tree/master/net-misc/keycloak-bin), directly using the official binary. If you need to compile it from source on Gentoo, it should be very cumbersome.