I used Gentoo myself back in the days where installing OpenOffice on it took about 12 hours (about 10 or more years ago). At some point I stopped using it because of not seeing any major benefit or speed compared to the distros that ship pre-compiled binaries. I have never tried it since then, but I doubt that it would really make a difference in most cases. According to distrowatch it is ranked as no. 45, which is not really impressive...
Supporting it requires a number of changes in our automated installer. We currently support apt-get, dnf and yum as package managers. If we would add emerge (I remember it being called like this), one would have to sit down and look at the corresponding lines of the installer (see here) and work out an elif section that sets the corresponding variables such that our installer can work with it. If Gentoo has no surprises on handling services, then that might be the only necessary change.
However, we are currently very busy with testing the new FTL engine and will move on to another large-scale change afterwards. So it is unlikely that one out of the developer test will be able to find the time to make himself familiar (again) with Gentoo and dig out the details that are needed. However, we may be able to give (some) support here if you decide to do it yourself. Also, we will be happy to accept a pull request into the installer code (or implement it in case you don't want to do that yourself) once it is in a reliable working condition.