Since groovy is good at parsing nearly anything, a search on how to parse groovy code will not reveal any good results - so I hope that the SO community is able to help :-)<
ASM should do most of the work for you....
Additional classes in Groovy Code likely wrap around ASM : -
Once you have a visitor, if there's a built-in adapter in the Groovy API, it could be "more or less" straightforward...