Actually, according to the May 2013 Thoughtworks Tech Radar at

http://www.thoughtworks.com/radar

In the Tools category, Gradle is "Adobt", while Maven is on "Hold". You can read the full report but I'll exert the relevant section:

"Language-based build tools like Gradle and Rake continue to offer finer-grained abstractions and more flexibility long term than XML and plug-in based tools like Ant and Maven. This allows them to grow gracefully as projects become more complex."

They had a more elaborate explanation in previous editions.