Gentoo is a source-based Linux distribution, so software is compiled as it installed (often at the cost of many hours!). As such, once linux-wiiu supports SMP, Gentoo is likely to be the distro that enables it, since the distro can be trivially compiled with SMP workarounds baked in.
A third-party image is available:
Once you have downloaded your image, proceed with the Install guide.
Huh? You want a stage3 tarball? You’re really better off using crossdev and compiling packages on something that Isn’t a Wii U. Have fun :)
Do try the SMP patches. SMP isn’t enabled yet but if the compiler breaks I’d like to know about it…
At some point in the future, linux-wiiu is likely to provide SMP-enabled binpkgs for Gentoo for regular users to enjoy without melting their Wii U. At that point this page will be updated with more actual instruction and guidance.
In addition to the linux-wiiu Matrix/Discord, YouTuber Cursed Silicon has been pushing forward Gentoo on Wii U development a fair bit, so his Discord has some knowledgable people in it.