The default nix devShell will have the packages needed to develop ocaml-gcloud:
nix develop '.#' # (or use nix-direnv)
dune build ...
If you've updated the .opam files in the project and need to recalculate the opam deps, run make onix-lock and refresh the devShell:
# From inside the nix devShell
$ make onix-lock
$ exit # (or nix-direnv-reload if using nix-direnv)
$ nix develop '.#'