How to do safe inserts using GORM for Mongo's low-level API?
This appears due to the GMongo library and how it patches the DBCollection object to handle passing of Map objects to the insert method and converts them. It assumes that all of the arguments of the insert method are Map objects and will then try to get the value property from the Map.Entry.
Looking at the source of Patcher.groovy from GMongo library you'll see the function _convert() that attempts to do this. It looks like a fork of the Github project with type check on the argument (either to see if it's a WriteConcern or to check if it's actually a Map before passing to the _converAllCharSeqToString) is necessary.
EDIT:
I created a pull request on Github for the appropriate code change, but as with all things Groovy, patching the class can also help. You can "patch" the WriteConcern class in your BootStrap.groovy to have a getValue method and that will allow you to pass the parameter in:
def init = { servletContext -> com.mongodb.WriteConcern.metaClass.getValue = { null }}