!!! All ebuilds that could satisfy "x11-drivers/nvidia-drivers" have been masked.
!!! One of the following masked packages is required to complete your request:
- x11-drivers/nvidia-drivers-525.85.05::gentoo (masked by: NVIDIA-r2 license(s), ~amd64 keyword)
A copy of the 'NVIDIA-r2' license is located at '/var/db/repos/gentoo/licenses/NVIDIA-r2'.

- x11-drivers/nvidia-drivers-525.78.01::gentoo (masked by: NVIDIA-r2 license(s), ~amd64 keyword)
- x11-drivers/nvidia-drivers-525.60.13::gentoo (masked by: NVIDIA-r2 license(s))
- x11-drivers/nvidia-drivers-525.47.06::gentoo (masked by: package.mask, NVIDIA-r2 license(s), ~amd64 keyword)
/var/db/repos/gentoo/profiles/package.mask:
# Ionen Wolkens <ionen@gentoo.org> (2021-10-09)
# Vulkan beta driver branch aimed at Vulkan developers for testing
# new features. Beside vulkan, it is typically behind the main branch
# and may be buggier or less secure. Only unmask if really wanted.

- x11-drivers/nvidia-drivers-515.86.01::gentoo (masked by: NVIDIA-r2 license(s))
- x11-drivers/nvidia-drivers-510.108.03::gentoo (masked by: NVIDIA-r2 license(s))
- x11-drivers/nvidia-drivers-470.161.03::gentoo (masked by: NVIDIA-r2 license(s))
- x11-drivers/nvidia-drivers-390.157::gentoo (masked by: NVIDIA-r2 license(s))

(dependency required by "x11-base/xorg-drivers-21.1-r2::gentoo[video_cards_nvidia]" [ebuild])
(dependency required by "x11-base/xorg-server-21.1.6::gentoo[xorg]" [ebuild])
(dependency required by "gnome-base/gdm-43.0::gentoo" [ebuild])
(dependency required by "gnome-base/gnome-shell-43.2-r1::gentoo" [ebuild])
(dependency required by "app-eselect/eselect-gnome-shell-extensions-20180306-r1::gentoo" [ebuild])
(dependency required by "gnome-extra/gnome-shell-extensions-43.1::gentoo" [ebuild])
(dependency required by "gnome-base/gnome-40.0-r1::gentoo[classic]" [ebuild])
(dependency required by "gnome-base/gnome" [argument])
For more information, see the MASKED PACKAGES section in the emerge
man page or refer to the Gentoo Handbook.

处理办法:
nano /etc/portage/package.license

添加
x11-drivers/nvidia-drivers  NVIDIA-r2