Does this actually work?
I'm seeing this in my logs and git things don't actually seem to work:

gitaly[2024]: time="2020-11-23T17:50:05+01:00" level=fatal msg="unsupported Git version: \"2.28.0\""

I didn't do much testing other than running the NixOS test, so it's definitely possible. Sorry about that.
I would port git 2.29.2 as a seperate attribute and change GitLab things to use that. Does that idea sound good to you?

I guess the test doesn't cover gitaly properly then, which is quite unfortunate. A different git attribute may be necessary, yes, since I imagine that's a bit of a jump for a stable branch.

In addition to running into the gitaly issue requiring git >= 2.29, I also ran into another issue when upgrading nixos 20.09.

If your system.stateVersion is < 20.03, then your gitlab install may be using postgres_9_6 (even if, like me, you had never explicitly enabled postgresql or chosen any specific version). Gitlab 13.0.x works with postgres 9, but 13.6.x seems to require at least postgresql 11. Thus, after upgrading, gitlab 13.6 fails to run one of its version upgrade scripts and fails to start.

The only remedy is to upgrade postgres from 9 to >= 11 using the upgrade instructions here: https://nixos.org/manual/nixos/stable/index.html#module-services-postgres-upgrading and then manually set services.postgres.package to >= postgresql_11.

Ideally nothing like this would ever be required when upgrading within a stable channel, but if that's not possible, at least an assert that the postgresql version is >= 11 in the nixos module would have saved me from an unpleasant surprise.

Just also FYI for the gitaly issue for anybody who's not already familiar with nixpkgs overlays, adding this to your nixos configuration will fix gitlab until #104896 is merged:

  # HACK: gitaly in nixos-20.09 requires git 2.29, which isn't yet in 20.09.
  # Remove when https://github.com/NixOS/nixpkgs/pull/104896 is merged.
  nixpkgs.overlays = [
    (self: super: {
      git = super.git.overrideAttrs (old: rec {
        version = "2.29.2";
        src = self.fetchurl {
          url = "https://www.kernel.org/pub/software/scm/git/git-${version}.tar.xz";
          sha256 = "1h87yv117ypnc0yi86941089c14n91gixk8b6shj2y35prp47z7j";
        };
      });
    })
  ];