Gradle is a build language, a Groovy-based DSL to define your builds. This means you don't have to develop your builds in XML, but you can use a proper language. You can add methods, refactor and create beautiful maintainable build files.
I think a big advantage compared to Ant is the support of plugins in Gradle. A plugin contains build configuration and custom tasks in a single package. You don't have to redefine a Java compiler task for each project, but you can re-use the compiler task from the Java plugin for example. This way it is much easier to re-use tasks across projects.
Ant is great because of the flexibility, we are not bound to a rigid predefined way of working. Gradle gives a choice. We can use the convention-over-configuration feature and really fast create a build for our project, but if the project is different (for example legacy source code directory structure) we can also adjust it easily.