You can create a debian environment within ubuntu or your favorite GNU/Linux distro (tested with Fedora) using pbuilder
http://wiki.debian.org/Diaspora/Packaging/pbuilder
Ubuntu specific instructions at https://wiki.ubuntu.com/PbuilderHowto
We'll use rspec_instafail as an example, substitute the gem you are looking for.
apt-get install git-buildpackage
You can find the code here
gbp-clone --pristine-tar http://anonscm.debian.org/git/pkg-ruby-extras/ruby-rspec-instafail.git
Follow instructions to run tests at,
http://wiki.debian.org/Teams/Ruby/Packaging/Tests
and to build a package from git use
git-buildpackage
To test in a clean debian environment,
sudo pbuilder build ../ruby-rspec-instafail*.dsc