How common are pool leaks in gunite pools?

More common than most folks realize.

Based on my experience over the last 2 decades, I would say that most pools have a leak of some type, with probably 3 out of 5 having a substantial leak!

This would be for pools in general and not specifically for gunite pools. Gunite shells don't leak unless cracked, but there are plenty of other places where a pool can leak.

36 years of remodeling and building gunite pools makes me disagree with the above answer. Very few pools have leaks with the most common place being in the skimmer throat. Also, after 5 years here in Nevada I have found more leaks in the light conduit than in 31 years in California. They use PVC conduit in Nev. and brass/copper conduit in Cal. Have a professional leak detection company detect the leak with dye and pressure testing to determine if the leak is in the vessel itself or in the Plumbing.