Reddit reviews Sir Gawain and the Green Knight

It's been published, and there are some inexpensive used copies on Amazon. If you want to see Tolkien's exact notes, try to get an earlier version (prior to 1968), since Oxford University Press did a little bit to modernize his notes.

By the way, it's not a translation. It's the Middle English text (which is literally unreadable by everybody but a scholar of Middle English), but you don't really need to worry about deciphering the actual poem. The cool part is that each stanza corresponds to Tolkien's notes, so if you have a copy of his later translation, you can pair the two and see how he formed his interpretation.