While we're a source-based distro, it can be useful to build reproducible binaries from Gentoo, both for Gentoo purposes (our own binhosts etc, or users' own), or even to reduce variance when debugging. We do tend to pick up fixes naturally from Debian and Arch as they've been working on this for a while, but there'll be some stuff we need to address on our end (either b/c not upstreamable, or needs some env var set, or some Gentoo-speciifc issue). Please file new bugs and have them block this one.
See also https://wiki.gentoo.org/wiki/User:OstCollector/Reproducible_Build.