Gem dependency for Deb packages

Hi, i want to package my application as deb, but i don’t know how to add
gem dependency to debian/control file. I look up from here.
http://docs.rubygems.org/read/chapter/20#dependencies

I defined gem dependency in .gemspec file like below:
s.add_dependency(gem_name)

But it defined for dependency in debian/control file:

Depends: ${shlibs:Depends}, ${misc:Depends}, ruby | ruby-interpreter

gem_name (>= 0)

gem_name was defined as comment line. Why? How can i defined gem
dependency?