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.
With Groovy 1.7-beta-2 and Ivy 2.1.0 being released it was easiest to just make a new point release of Gant, hence we now have Gant 1.8.1. See http://gant.codehaus.org/Prepackaged+Distributions for details of getting things.