Installing itch on Gentoo
At the time of this writing, there is no PKGBUILD available for itch in any overlay, that we know of.
However, I (Amos) hold a special place in my heart for Gentoo, having used it for several years during my college days, and although I don't have the time to maintain one, if someone feels like maintaining a PKGBUILD for itch, I would be happy to provide support.
If you're interested, please open a new issue to start a discussion!
We ship portable archives of itch at https://github.com/itchio/itch/releases which you should be able to unpack and run without much trouble, provided you have the few X and gtk libraries required.
Alternatively, you can always follow the Getting started guide to run it as if in dev.