Glad that Geddy is working out well for you!
There's no reason you can't use Grunt (or Make, or Rake, or any other build tool) with your Geddy app. Internally Geddy is using Jake for all the generators, and stubs out a Jakefile for your projects, so the path of least resistance is to use Jake.
Bottom line, you should use whatever you like the best. :)
M.