South Gully is the most defined feature on the face, but as it terminates on the ridge some way below the summit, it doesn't see the attention it probably should. It's actually 2 gullies running in parallel, separated by a shallow rib. In ascent, the left hand line twists a little and is shorter; the right hand line is straighter and is the true line of the gully.
The left hand line (in ascent) cuts to the left towards the ridge line at the top via a narrow gully. You are most likely to find a small bouldery cul-de-sac that requires a couple of metres scramble to get onto the ridge itself, although it's not improbable that enough snow would allow a complete snow covered ascent/descent. If this was the case, you would be able to ski off the summit of Ben Lui some way into Coire an t-Sneachda, and then into Coire Gaothach via this line.
The right hand line (in ascent) goes higher up the ridge, and is the true line of South Gully. From below, it appears to end in a headwall, but there is a narrow gully on the right that may allow a way through and onto the ridge. I 've not been to the top of this line, so you will need to check it out yourself! I'd be interested to know if there is an exit onto the ridge above that allows a complete descent.
Approximate Start Height: 1000m
Approximate Descent: 300m
General Aspect: Northeast
Climbing Grade: I