The GroovyPagesTemplateEngine, which is responsible for rendering GSPs, isn't usable outside of a webrequest.
Very common use-cases for rendering GSPs independently of a web-request are for example:
- Sending HTML notification emails asynchronously (for example not possible with the MailPlugin at the moment)
- A job that renders some static HTML files once the day
Of course you can use Groovy's SimpleTemplateEngine for these use-cases.
But you can't use custom tags and SiteMesh functionality which is a very big restriction compared to the GroovyPagesTemplateEngine.