But out of curiosity, where is Christmas Gully? Looking a t Gaia here I see two prominent gullies on the Strachan side, one climber's left of the chair and one further away to climber's right which joins up with the "Collins" run. Is it one of those?
No. It is gully on west side, between north and south peaks. You hike HSCT for about 1/2 hr to Strachan Meadows
That is Strachan North (higher) sunlit upper center left. Here you leave HSCT and go right and up.