You were very close. Keep walking up the hill past the boulders, this is where the guidebook helpfully points out that you will get lost without giving any hints as to how to get found. It envokes images in my mind of simon throwing up his hands in dismissal and wandering off to the pub.
The gallery is kinda just over the hill past the boulders. The cairns should lead you there. there is a bit of a flat rock slab at the top near the spectacular canyon slot that is not mentioned in the guidebook. It is easy to walk past it for some reason. walk through it and you are there. Persevere, It is there and it is steep and hilariously fun!