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