I've used Gradle before and must say, I'm not really a fan of it, but it does have its place.
Even though a lot of Java developers hate maven, what happens when you tell a new guy to get himself setup using Maven? He would checkout the code and simply run mvn clean install and he'd be up and running, no matter what project he wants to work on, to compile, it's always the same commands.
Gradle on the other hand you can complicate builds a lot and that's the same problem I have with Ant.
Arguments I've heard before is that you can't do custom things with Maven which I usually prove wrong by writing a reusable Maven plugin in 5 minutes using Java code then hooking it into the process.
Wherever I go and see Ant still being used, I convert them to Maven, if they're using Eclipse / Netbeans to build (the horrors, especially when people are using different versions of the IDEs), I write a Maven pom file, if I see Gradle, I overlay Maven as well so you could use both.
From experience, the majority of places I've been to were still using Eclipse / Netbeans to do builds or Ant and adding Maven just made it easier to do automated builds. Gradle I've only seen at one place and it was a mess to work with, it had 10 different custom built commands which nobody knew how to use properly unless they were the ones who wrote it, Maven I've seen a lot more than Gradle. Most of the open-source frameworks I use are using Maven (except for Android).
My opinion. Maven is the better tool in 90% of cases, it hides a lot of the complexity, it forces everyone and every project to work in the same way (so a big win for productivity), it has a very well designed plugin system. Gradle on the other hand is like a 100-piece swiss-army knife when in most cases you only need a knife.