How can I use Gson in Scala to serialize a List?

I was hoping to use Scala and Gson together. It seems to mostly work, but when I do something like this, it treats the list as an object, not an array:

case class MyType (val x:String, val y:List[SomeOtherType]) {
    def toJson() = new Gson().toJson(this)
}

And my JSON turns out something like this:

{
    "x":"whatever",
    "y": {

    }
}

Normally Gson converts lists to arrays. I'm sure this is all because Gson doesn't know about Scala's collection classes, but any ideas on what I can do to make this work? Or other suggestions using Scala-native JSON libraries?