Compiling obs-studio with the nvenc use flag enabled does not pull nvidia-video-codec, which causes nvenc encoding to not work. Emerged nvidia-video-codec and it straight out worked, so I'm supposing it is a bug.
Interesting. I haven't had this package installed myself, according to my logs, and I don't have NVIDIA hardware on my main machine any longer. I wonder what changed. In any case, will look into it. Thanks for reporting!
My two cents: At least in my machine pulling nvidia-video-codec is not necessary. To enable video (de)en-coding it is sufficient to enable "nvenc" USE flag in obs-studio AND "uvm" USE flag in nvidia-drivers. My setup: x11-drivers/nvidia-drivers-455.28-r1::gentoo was built with the following: USE="X driver gtk3 kms (libglvnd) multilib static-libs tools uvm -compat -wayland" ABI_X86="32 (64) (-x32)" media-video/obs-studio-25.0.8-r1::gentoo was built with the following: USE="alsa fdk jack nvenc pulseaudio ssl truetype v4l vlc -imagemagick -luajit -python -speex" ABI_X86="(64)" PYTHON_SINGLE_TARGET="-python3_6 -python3_7" and I can (de)en-code with NVENC with no problems.
nvidia-video-codec hasn't been necessary for me either, nvenc works fine with obs Furthermore, USE=uvm was removed and no longer necessary to get this to work (perhaps was related to your issues). Feel free to say if this is still an issue for you and I'll look further into it.