Apparrently, a "cat-sized hole" is also a "Zoë-sized hole". Swear to God, she can get through an 11" x 6" space in the baby-gate. We've shimmed the doorframe so it's now an 11" x 5" hole, and we still think she can get through. Any narrower and the cats probably can't get through.
Hmm.
Measure the cats heads. Any cat can go through a hole where its head can go through. Pick the biggest cat head and see if the dog head is bigger. Dogs can't do the trick (their chest is too big or their legs are too long) or are too stupid to learn the trick.
That's true. Even Grendel will fit through a hole that's only as big as his head. It's a tight fit, and he sorta oozes through it after a point (EWWW!), but he can (and has, unfortunately) done it.
Many short-legged dogs also really suck at getting over a low barricade. Even if your older cat isn't capable of gymnastics, it is likely that he can get over a higher hurdle than Zoe can. Our dachsund mix (Sparky) is stopped by a 6-inch-wide board placed across a doorway. Try placing your gap up high and see what happens.