The arch wiki is a good substitute, but the gentoo wiki when it was still around and at its peak was amazing.

But I agree… Gentoo is not quite keeping up with a lot of details. Like experimenting with refind, dracut, efistubs, I felt I was in the dark a lot of the time. I ended up making very few mistakes, because the distro is very good at working for special cases even if all the details are not explained. Still my favourite distro.