I really like plone as a CMS, but its base is in Python. I would like to know if there is a CMS that has Grails as a base.
Wrong comparison. Python is a language. Grails is a framwork.
Plone (app) --> Zope (framework) --> Python (language)
? (app) --> Grails (framework) --> Groovy (language)
Don't know too much about Grails but maybe this is what you're looking for... http://sourceforge.net/projects/gocms/
Probably not in the same league as Plone, but then few CMS's are.
You might want to take a look at Weceem CMS, which is open-source and uses Grails as the foundation.
There is also Codice ( http: //github.com/neodevelop/codice/tree/master ) and Countour-CMS (http: //www.seo-web-services.co.uk/a/contour-cms ) But none of these are production ready.
You can always integrate PHP packages like Wordpress and Plone into grails via the PHP plugin, which uses Quercus.