After about 4 weeks for part time research, trial-and-error, and countless night digging Grails source code, I finally finished the my first useful grails plugin.
It's really a search plugin, using lucene as the engine. It still has many basic features to go before I can call it barebone complete, but what the heck I'm using it on production system.
I will be releasing the source code and licensed it same as Grails's licenced term, which is Apache 2.0.
Coincidentally Maurice Nicholson released his Searchable Plugin 0.1 last night.