How to add custom python package?

Development
2

attribute 'gvm-tools' missing is the key thing.

Your overlay adds gvm-tools as a top level package. You need to do something like

python35Packages = {
  gvm-tools = super.callPackage #...
};

I think Nix will merge the sets in the overlays.

Even better, if it’s appropriate, would be to add your package to nixpkgs and use the instructions in Nixpkgs 23.11 manual | Nix & NixOS to build and install locally until the PR is merged.