Can I define my own package or gem dependencies?

You can define a bunch of Ubuntu packages and RubyGems you want to install on instances of a specific role. Go to the cloud's detail view and select "Dependencies" for the role you want to add packages to.

You can specify a version for RubyGems, otherwise the most recent one will be fetched. For Ubuntu packages we'll always be referring to the version that's most recent for the choosen OS, including updates.

If you have to add many packages and gems, consider writing a custom Chef recipe that installs all of those.