Transforms don't work at all for applications compiled with GMaven. GMaven provides a custom class loader to avoid pollution of the compile class path. As far as I remember, this class loader doesn't satisfy the assumption made by the transform implementation that it can load a transform and cast it to ASTTransformation (i.e. the instanceof check fails). I tried to address the root problem in http://jira.codehaus.org/browse/GROOVY-3169, but it was decided that it was too late for such a major change to make it into 1.6. However, some workaround has to be found so that transforms do work under GMaven by the time Groovy 1.6 is released. Otherwise, many Groovy projects won't be able to use transforms, possibly for a long time to come.