Gradle support - IntelliJ doesn't appear to understand "ext"

I define project properties like this:

project.ext.playn = [

    version: '1.8.5'

]

and then use them in subprojects like

compile "com.googlecode.playn:playn-java:$project.playn.version"

Now, I'm not actually sure if that's how you're supposed to pass properties around in gradle (I found the documentation to be kinda vague), but it's valid and it works.

IntelliJ doesn't like it though (it can neither resolve "ext" nor any attributes created that way).

On a slightly related note: It doesn't recognize "attributes" in "manifest" either.

This seems kinda weak considering "improved gradle support" was one of the feature lines for 13.1 - am I missing something? Is there some additional something I have to enable or set to make this work properly?