added a comment - - edited
I just found this issue by searching. I'm trying to create my first groovlet in Eclipse, using Groovy 2.0.6. As lee said on the mailing list http://groovy.329449.n5.nabble.com/How-to-create-a-groovlet-in-Eclipse-td3351347.html, I'd say start simple.
I'd like to be able to create a web project in Eclipse with a Groovy nature. Minimum requirements would be that (1) Eclipse recognizes the project as a web project, with a WEB-INF folder, web.xml and JARs in WEB-INF/lib; and (2) Groovy recognizes the .groovy files and provides it's normal assistance. I understand that the Groovy plugin currently doesn't recognize the groovlet DSL; would be better if it did, but can live without that as a first step. If the plugin can be updated to still provide assistance on the script parts but ignore the HTML parts, that a good start.
[UPDATE] Many hours later...
I've made some progress. I started by creating a dynamic web project. Once the web project was created, I was then able to right click on the project name and a menu item showed allowing me to "change to Groovy project". Fortunately, that action kept all the web project artifacts: webroot, etc. Because this was intended to be a quick and dirty project, I originally put a .groovy script under WebRoot, containing all my logic as well as the HTML. (I gave up on MarkupBuilder and resorted to inline HTML; I was having to convert HTML tags in my head anyway, so I decided why bother.) That didn't work very well; context assist is essentially non-functional. That approach is parallel to JSP model 1, which is strongly discouraged anyway.
So, I then moved all the logic into a Groovy class under the src directory. The groovlet now contains just the fragments need to retrieve and set parameters for the page display, as well as the HTML. Context assist works great for files in the src directory. So that is all good, and I can iterate development very easily with the built-in Tomcat 7.
But it is far from perfect. Since Eclipse doesn't know about groovlets, it doesn't copy all the required libs into the WEB-INF/lib directory. I had to do that manually. That's where enhancements to the Groovy plug-in would help groovlets. When I'm all done, I'll do all that in build.xml and ivy.xml.