I'm going to have to think on this one, but here are my initial thoughts;
1 - Can you circle in the photo what was rebuilt vs what is original?
2 - Can you show exactly where the water is leaking? If it's just the dark area right of the steps, it seems minor.
3 - Is water actually weeping through the mortar? Or coming underneath the wall?
3a - Are you certain there is no crack towards the bottom of the wall allowing water to pass? This seems to be a more likely scenario to me, especially if you are just getting water from the area by the steps. If that's the case, you can chase out the bad area and patch with hydraulic cement.

A couple points I need to make;
A - Gunite itself is not waterproof. That's why plaster is added as a finish.
B - If it is a weepage (don't know if that's a word, but it works for me) issue, it's going to be the mortar, not the stone. I say that because a water sealant product is going to be surface specific. No point in buying expensive stone sealant when mortar sealant is what you need.