This week's book giveaway is in the OCPJP forum. We're giving away four copies of OCA/OCP Java SE 7 Programmer I & II Study Guide and have Kathy Sierra & Bert Bates on-line! See this thread for details.
Grails uses GSPs which are really just JSPs but with a few more features because of being able to write groovy code in EL. Since JSP 2.0, libraries like Velocity don't really gain you anything. Why would you want to use Velocity with Grails?
There is no support, that I am aware of, for Velocity in Grails.
The only benefit I can think of for using a separate templating language for the view, e.g. Velocity or Freemarker, is that you can test the views without running the entire application. However, if I wanted to unit test the views I would rather use Java or Groovy code than a template language.
After a quick Google I think it is more likely that freemarker will be supported before velocity, if either of them are on the plan.