(root)# apt install nix-bin

It may be required to set up nix manually.

(user)[~]$ mkdir -p .local/share/nix/root/nix
(root)# mount -o bind ~user/.local/share/nix/root/nix/ /nix

The package description is in a git repo hosted on github. (TODO: find mirror.)

(user)$ cd /some/where
(user)$ git clone https://github.com/nixos/nixpkgs
(user)$ cd nixpkgs

To build the packages (from master and/or for development purposes), do

(user)[nixpkgs]$ nix-build . -A gnucap-full

Alternatively, select the official unstable repo and use binary packages

(user)[nixpkgs]$ git switch nixpkgs-unstable

The packages are now available inside nix-shell.

(user)[nixpkgs]$ cd
[..]

(user)[~]$ nix-shell -p gnucap-full -I nixpkgs=/some/where/nixpkgs
[nix-shell:~]$ gnucap
[..]
gnucap> _