A Groovy-based build system that uses Ant tasks, but no XML.
Gant is Groovy Ant Scripting
Gant is a tool for scripting ant tasks using groovy instead of xml to specify the logic. A Gant specification is a Groovy script and so can bring all the power of Groovy to bear directly, something not possible with Ant scripts. Whilst it might be seen as a competitor to Ant, Gant uses Ant tasks for many of the actions, so Gant is really an alternative way of doing things using Ant, but using a programming language rather than XML to specify the rules.
Gant isn't really a "build framework"
Gant is just a lightweight facade on Groovy's AntBuilder. It is just a way of scripting Ant tasks using Groovy. Gant can be used to do build tasks, but it doesn't have the integrated artifact dependency management, project lifecycle management, and multi-module/sub-project support that a fully fledged build framework should provide. gradle on the other hand is a complete build framework based on Groovy and ivy. If you just want to do some Ant task scripting then Gant is probably the tool you need, but for replacing Ant and maven as build frameworks (so as to get rid of all the XML and use Groovy), then you probably need to consider Gradle.
