I actually spent a part of the weekend re-evaluating Scala build tools for the who-knows-how-many times now.
Gradle did come up and I was astonished to discover Groovy is now over its performance problems and may very well be a worthy platform to develop web services on!. Only if Groovy had a clear road map and profound backing it would have been the true "Scala killer".
I think it's worth giving Groovy a second chance by its own, and I've set up some time to do that.
And lastly - after looking at gradle as a build tool for Scala - it looks OK, however I don't think I will ever withstand all of the hate and incompatibility from the Scala community because I've deviated from SBT (which is another problem by itself).