If you are on a Mac, you can use TextMate and install Groovy / Grails . You will have code snippets, this is a kind of completion.
If you are running Windows / Linux / Mac, you can use Redcar , which was created for (and in) Ruby but has Groovy / Grails Support. It also has code snippets.
I highly recommend finding out that you are not using object validation and dynamic code completion in a dynamic language to avoid IDE dependency and dependency.
I personally prefer an unobtrusive code editor that allows you to directly process text code without interference, which allows us to make mistakes earlier and better understand the language (and structure). I use it even in production work.
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