Gant is a Groovy-based build system that uses Ant tasks, but no XML. It can be considered an alternative to Ant.
Gant is a build tool for scripting Ant tasks using Groovy instead of XML to specify the build logic. A Gant build 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 builds using Ant, but using a programming language rather than XML to specify the build rules.