Is it possible to add a :git source to a .gemspec as dependency?

Question

I got multiple gems dependent on each other, say:

core - base stuff
module 1 - uses core
module 2 - uses core

and I'd like to use them in application 1, 2 and 3.

application 1 - uses module 1
application 2 - uses module 2
application 3 - also uses module 2

Now the thing I want to do in module 1 and 2's .gemspec is:

Gem::Specification.new do |s|
  s.add_dependency "core", :git => "git@mygit.com/core.git"
end

Is that possible in some way? This code above gets me 'Illformed requirement' errors.

Solution to get it working is to add gem "core", :git => "localpath/core.git" in every Gemfile of application 1, 2, 3. This is obviously something I'd like to avoid, I need the gems to retrieve their dependencies from a source given in the gem itself.