In working with Grails and MongoDB, I found myself building out some data type documentation I wasn’t able to find.  For your coding pleasure, please find…

Groovy Type Mongo Type
Integer Int32
Double Double
Long Int64
Float String
Short Int32

Grails is great, but the documentation is limited and the dates/versions of the references matter a lot (it seems Grails has been through many versions and many recommended approaches, not all of which are backwards-compatible).  The above mapping is for Grails 2.4.4 and the mongodb plugin 3.0.2, atop MongoDB 2.6.3.