Add this flake as an input:
inputs.nixvim.url = "github:gabrielemercolino/.nixvim";
Then you can install the package as you like:
# in configuration.nix...
environment.systemPackages = [ inputs.nixvim.packages.x86_64-linux.default ];
# or in home.nix
home.packages = [ inputs.nixvim.packages.x86_64-linux.default ];
To test the configuration simply by cloning and runing the following command:
nix run .