gem dependency (with no args) should show you all gems from current system with their dependencies.
bundle exec gem dependency will show you for the current Gemfile
Edit:
You can also do gem dependency -R (or just dep instead of dependency) if you want to find out which gems use specific (or all) gems.
For deeper dependencies I’d parse output (regex maybe?) of first gem dependencies, pick gem’s names and call gem dep on each of them, but that’s just a loose idea.