The current supported debian-kernel on funtoo portage is:

sys-kernel/debian-sources-5.10.46_p4

This kernel version is not compatible with

nvidia-drivers-460.32.03-r2

which only support:

# Maximum supported kernel version in form major.minor
: "${NV_MAX_KERNEL_VERSION:=5.10}" 

This causes, that after updating to the new debian-sources kernel, that nvidia driver can't be compiled.

Possible solution:
Update nvidia-drivers to the new version ( 470.63.01 )