How do I install nix os without a usb on gentoo?

How do I install nix os without a usb on gentoo?
I really do NOT want broken deps.

If you don’t want to wipe the existing OS drive, you should be able to do it by just installing Nix the package manager on Gentoo, then proceeding with more or less the normal installation instructions (nixos-generate-config; nixos-install etc.).

If you’re looking to install NixOS in place of the existing Gentoo install, you might have success with the kexec images.

like, porting the installed gentoo packages to nix

Not sure I follow you. If you’re looking for some solution that would generate a NixOS config that would include everything you currently have installed on your Gentoo system, I don’t think such out-of-the-box tool exists.