Nvidia and rt-sources has a long-standing conflict. The current nvidia-drivers version that I can get working with rt-sources (currently 4.13.10_p3) is nvidia-drivers-381.22-r1. I am unable to compile any higher version than that against rt-sources.

If anyone develops a fix for this, please let me know, so I can incorporate it into Gentoo Studio and share it with other Gentoo users using rt-sources.