Grails is great for building a REST API. Especially the latest version. It makes it dead simple. I do SPA frameworks on the front and JSON APIs from the back.
Grails is great to get things done quickly (scaffolding). But if you want to write vendor independent code, Grails makes your life unnecessarily hard by providing a full stack from Database to UI. Somewhere something leaks into the business logic. This is less the case if you can pick and control exactly what Database and REST technology you use. Of course you can ignore the DB part in Grails and plug your own thing, but then, why should I use Grails in the first place?
