GQuick does discuss what Grails is best for, which (in my somewhat biased opinion) is web applications, large and small, that target the
Java Virtual Machine. Grails produces a Java .war file which can be deployed to any standard Java web container (
Tomcat, Jetty,
JBoss, Glassfish, WebLogic, etc.)
I don't cover using Grails for desktop development, however you could conceivably use Grails to build the server-side component of a distributed application that included a desktop component. As an example, at the recent SpringOne 2GX conference in New Orleans, Jeff Brown built a desktop application in Griffon (the Groovy Swing MVC framework) that accessed a Grails application via a webservice.
Dave