All your own code in Grails can be 'plain Java' if you want (or pretty darn close to it). The nice thing is the Java people who want to stick with Java can do that in Grails, or you can go pretty deep in to Groovy if you want.
Grails uses Maven and Hibernate, but you don't need to become masters in them before you start being productive.
This is why I like Grails so much - I've been able to be productive from a very early stage. The more I learn about the under the hood tools, the more productive I become, but I didn't need to understand or configure Hibernate, Sitemesh, Maven or Tomcat to get going. Now that I know more about them, it's even better, but it's great to jump in and have a working app early on (even if it's pretty basic).
Perhaps others learn differently, but Grails has been a great 'on ramp' to Java for me.
http://www.yes-no-cancel.co.uk/2009/05/13/building-go-test-i...
All that said, Strachan isn't really involved in Groovy and hasn't been for many years, so I'm not sure his views on the language or its appropriateness is of that much concern. A whole new team of people have been running with Groovy for years, and continue to make it better and more useful to people.
Scala is different enough from Java that syntactically that it's a hard sell for many. Groovy is far more Java-ish for Java devs to pick up, and as such I think it will continue to have a growing userbase for the foreseeable future (but I'm biased, as I run groovymag.com!)