http://en.wikipedia.org/wiki/ISO_3166-1 would seem to suggest that you just use "gr"
as all of the other ids used in the plus language files are the two digit codes listed there
The expanded codes to include country and langauge only appear to be used when there are two different versions of the language included (such as zh-cn and zh-tw for the two versions of chinese spoken in china and taiwan)
hope this clears things up for you