Funtoo actually isn’t really gentoo-based, I think? It seems like it’s more of a gentoo variant, because its definitely more different than gentoo than just putting extra stuff on top of the base distro. It’s main purpose is to build gentoo, and it’s a stable release instead of a rolling one like gentoo. https://www.funtoo.org/FAQ It seems like it’s based off of a very old version of gentoo(2019), and has diverged a lot more from gentoo since then, for example, I’m pretty sure that the gentoo ebuilds no longer work on funtoo, maybe they do but require some work sometimes? But for more traditionally gentoo-based distros, there’s stuff like calculate linux and redcore
The most "discord like" and most well known one atm is the Matrix protocol. I won't bore you to death with how it works, all you need to know is, if you wanna use Matrix, you need a Matrix client. The most popular and complete one is Element.
Though whenever the topic of discord alternatives comes up, I always gotta bring up IRC, because it's been around since the 80s, can be ran in a toaster, will never turn corporate, and will ALWAYS be here with us until the end of time!
And lastly, another cool one is Tox. This is the one you use if you REALLY want peak privacy. It's peer to peer (so there's no central server where the messages have to pass through) and it's end to end encrypted. It supports text, files, and even voice chat! However, it is a little bit rough around the edges, so it may not be easy to convince your friends to move to Tox.