I've seen companies using Gentoo (with a lot of tooling around it) in specialized context.
Particularly in military contracts, Gentoo brings you several things:
1) you have a full audit trace from the compilation, and in paranoid situation, it's a huge plus.
2) it permits you to minimize the OS footprint quite a lot (feature flags), which helps improving security (less exposure).
3) it permits an high degree of customization, including the combination of software versions used.
4) it's relatively easy to import a patch and maintain an overlay yourself.
We have to keep in mind the constrains of Military contracts: The systems tends to be quite complex, validation is quite lengthy, updates are far in between, they tend to use specialized/weird/not powerful hardware which might need some tweaking, and the level of traceability is generally quite high.
