This is not possible, and likely never will be because it would be rather heavy-handed for RubyGems to allow gem developers to require that users have a specific version control system installed to access a gem. Gems should be self-contained with a minimal number of dependencies so that people can use them in as wide an array of application as possible.
If you want to do this for your own internal projects, my suggestion would be to use Bundler which supports this quite well.
与恶龙缠斗过久,自身亦成为恶龙;凝视深渊过久,深渊将回以凝视…