Before Maven, there was Ant. Ant is a fancy Makefile written in XML that makes it very very easy to write build scripts that are not reproducible and rely on a particular structure and set of tools on the computer of the person that wrote the build file. At some point people realized the madness and wrote a new build tool that would only use declarative syntax, and would provide reproducible, contained build bliss. And lo and behold Maven came to be.
Later XML came into disfavor, as things do, and thus by association Maven as it uses XML. So they created Gradle, a tool deemed superior by the mere virtue of using a Groovy DSL instead of XML.
Here's my opinion about Gradle :
1 - the syntax is poor. Merely not using XML does not imply that it is fun to write Gradle build files. XML is verbose but very well specified so when you need to write some configuration it is a bit tedious but all in all very easy. On the other end Gradle uses a custom DSL that is poorly documented. Usually the best documentation there is are example files, so it can be very hard to figure out what to do in uncommon cases.
2 - plugins can extend the DSL. That means there is no single document to learn to know how to write Gradle build files, you also need to learn the DSL for the plugins you use. Maven developers went as far as specifying a standard plugin documentation format so that when you need to look for the syntax for a particular plugin you can do so fairly easily. Not so with Gradle, documentation quality is inconsistent and hard to find. The Android plugin seems especially bad in that regard.
3 - it killed the dream of declarative builds. Gradle allows to run Ant files directly and is simply Groovy under the hood so build scripts can quickly become ad hoc messes. Not what I call progress.