Just to expand on this, the reason for that is that Nixpkgs doesn’t provide derivations for specific versions of Ruby Gems. Instead, it provides functions for building arbitrary versions of Ruby Gems, since different programs will be locked to different versions, and would be unhappy with Nixpkgs supplying different versions.
I’ve thought about this too, in particular because it would be useful for nix-shell-shebang scripts, like ghcWithPackages. I haven’t yet come up with a way it would be useful or maintainable though, for the reasons you describe.
Perhaps, rather than exposing packages, we could support such a use case in a similar way Bundler does with bundler/inline… We could have some library you could make available to your script that would essentially run Bundix and then apply the resulting environment to your script. I’d need to think that through a little more to determine whether it would even be possible, but my instinct is that it should be.