Using the same basic function as chruby_auto I created gemsets based on the Gemfile's parent directory name and replace the entire GEM environment variables with that single gem location.
Logically assuming all Gemfiles have a corresponding gemset means shim-less, bundler-managed gems.
This needs tests and could be refactored into a shared function that chruby_auto could also use.
This is a proof of concept.
You might want to look at chgems. It sets GEM_HOME to $PWD/.gem/$ruby/$version to make cleanup easy and spawns a sub-shell instead of auto-switching. Also, I don't think it's wise to use gemsets for every bundler project.
Scratched my own itch. For others with this itch: https://github.com/daytonn/auto-gemsets
Closing now that auto-gemsets is available to users.