GNER doesn't actually use a code- It's all under the normal value single fares stage of ticket bookings.
On the examples i've checked out so far (York-Kings Cross, Inverness-Kings Cross) , the cheapest GNER single fare is literally just £2-3 more, and I've not seen any of the cheapest GNER fares/DM fares available yet, although i didn't look mid-week.
Actually they do, the code is MALLARD225. There looks to be virtually no availability at weekends, although there's quite a bit in the week. Also, the fact that you can't travel until after the half term holiday is useless for me!