Why would you need a decompiler to write a debugger for Groovy? The groovy compiler supports including source code line numbers and variable names in compiled jar files (just like Java). As a result you can use any Java debugger to debug Groovy.

Also, why would you expect that Groovy can be compiled and then decompiled to Java? This doesn't even work 100% going from Java to Java, and Groovy is going to have idioms that doesn't map easily to Java idioms.