With this sort of thing it's hard to determine what is actually causing that error. You didn't specify the source of that error message but I am guessing that it must be coming from the kernel module. What follows is not intended to diagnose that error so much as to remove the nVidia binary driver that you have been using and replace it with the one from the Portage tree.
You should probably close X entirely and do this from the console.
If you install the nVidia binary drivers manually, the installer is in the form of an executable script with a name like NVIDIA-Linux-x86-190.42-pkg0.run. If you don't have this, you can download it from nvidia.com. It has its own uninstall command. You can run it with a command like this:
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localhost ~ # ./NVIDIA-Linux-x86-190.42-pkg0.run --uninstall
At this point you may want to make sure the kernel module is not still loaded in memory:
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localhost ~ # lsmod
Module Size Used by
nvidia 8872171 52
localhost ~ # rmmod nvidia
localhost ~ #
Then you can install the one in Portage:
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localhost ~ # emerge x11-drivers/nvidia-drivers
This will build a new kernel module from source and install the binary GLX driver.
At the end of that installation procedure, Portage itself will execute the command "eselect opengl set nvidia". This will establish several symlinks that tell your system which libGL/libGLcore to use. It'll know where to find those once you are using the nvidia-drivers from the Portage tree. When you run X again, it should automatically load the kernel module Portage just built.
Hopefully that will get everything working for you.