To use textmate gem would be rather simple. But one thing is a bit tricky. textmate depends on Rubygem and thor. I believe if one wants to write a general GetBundle command one should make usage only of default libraries. By my opinion GetBundle is an essential piece of software. I guess it could be a bit annoying for people who want to use this GetBundle to install several other Ruby libraries in beforehand.
--Hans
Then just include the thor gem in GetBundle too. "freeze the gem" as they say.