People on arch had an issue with the order of kernel modules loading messing up the functions. Check this if you didn’t try already:
Also several distros had an issue with iio-sensor-proxy existing as a service but being unable to fully start with similar errors, due to some but not all requirements missing. I’m using openSUSE personally, and waiting for the following modules to drop into the default kernel, because I can’t be arsed to compile my own:
There’s also an user with nixos, who shared their working config, maybe you can adapt this to configure the same changes on gentoo:
If you are on gentoo, I expect you are using the lates packages. So the iio-sensor-proxy bug that has been fixed upstream about a week ago will not affect you. Do not apply any udev rule patches suggested because of this bug.