Okay, the first things you will need in your POM file are the GMaven mojo and the GMaven runtime. The dependencies for the mojo seem to be a big buggered out of the box, so you will have to tweak the GMaven runtime dependencies. Here is how you do it:
Next is your build configuration. Eclipse will need to be able to find your Groovy source files, so you will need to add your Groovy source as an explicit resource directory.
The mvn:eclipse plugin will also need to add the Groovy nature to your project.
You will also need to add directives for the GMaven plugin. This is where you will have to do a manual tweak: when using mvn to build your project, gmaven generates some stubs for your groovy code before the java code compiles, so that any java->groovy dependencies will be fulfilled. The Groovy code is then compiled and will overwrite the .class files from the previous step. This breaks things in eclipse, since if you attempt to execute code in your workbench, eclipse sees the stubs and somehow their .class files are what end up in your binary output directory.
Therefore, if you want to execute a gmaven project from Eclipse, you simply need to comment out the stub generation directive and delete any generated stub classes. This is what it looks like:
The result is a groovy/java project that can be managed with maven but can be transparently developed (and more importantly, debugged!) in Eclipse.
There is one minor issue that has come up however: GMaven 1.6 stub generation is broken for enums: it generates public constructors, which do not compile. I submitted an issue to codehaus (GMAVEN-51), and apparently the workaround is to use the gmaven 1.7 runtime. I haven't tried it yet, since things are working okay for us thus far.
So, it does seem like there is a bit of a problem with stubs being recognized by Groovy-Eclipse when they should not be. The solution would be to generate the stubs in a directory that is not seen by Eclipse, rather than in the default output directory. But not knowing much about GMaven (or Maven for that matter) works, I don't know how feasible this is. Can anyone think of a better solution?