Haven't used grails, but back in 2012, JRuby used to generate so many in-memory classes, the class space would run out and we didn't really have a garbage collector for that. It came a few years later. Things might be better with JRuby 9000 and java 1.8+ these days. I presume Grails didn't suffer such problems.

Note: I see that torquebox was discontinued. It was a simple way of getting your rails project to work with JBoss and had many nice utilities like infinispan and stomp. Good old days.