Have a look at how they do it in cli.rb
def locate_gem(name)
spec = Bundler.load.specs.find{|s| s.name == name }
raise GemNotFound, "Could not find gem '#{name}' in the current bundle." unless spec
if spec.name == 'bundler'
return File.expand_path('../../../', __FILE__)
end
spec.full_gem_path
end