Does Gentoo use the Bootstrappable Builds process yet? ISTR someone was working on it.
Windows/macOS/etc are pretty irrelevant if you don't want to trust binaries, because most of them don't come with full source code. People who care about this stuff aren't even going to consider proprietary platforms.
In any scenario where you would do a full-source bootstrap, you would be reviewing the code for each step of the process, or deciding which reviews published using crev or similar are trustworthy enough for you.