The Faq Gap is a break in of High Mountains in the ring of Impassable Mountains surrounding the Kingdom of Faq. Through this hex of High Mountains, (and only through this hex) a group of flying units or mountain-capable units may enter or exit the kingdom. It forms one of the natural choke points into and out of the kingdom. (The other being the Tunnels through the mountains).
"Jillian informed Ansom that Faq could only be accessed by air, or through tunnels. Unless she was exaggerating, not even Mountain Units like the Western Giants can reach Faq through the Gap on foot, which suggests that the Faq Gap is composed of High Mountains rather than ordinary Mountains. Perhaps mountain capable units can't pass through high mountain hexes on foot (unless they are also tunnelers).
Impassable Mountains are a Terrain Type which cannot be passed even by flying Units. It's clear that there must be at least two places in the Mountains surrounding Faq where fliers can pass, because fliers from Transylvito used to pass through [Faq]] airspace on the way to somewhere else."
Jillian might have been speaking generally, or might not have known about mountain-capable units.
Though Jillian traveled widely as a mercenary.
Or, the gap is regular mountains. Either way, we know the giants can cross the mountains/high mountains (though not the Impassable ones). We also know the Gap is the only one of its type because a) Jillian calls it "the" gap, and b) a warlord is sent to defend it, and no other warlord (3 available) was sent to defend another such gap. As far as Transylvito units passing over the kingdom, I'm not sure that "Faq airspace" doesn't cover a greater area than the impassable mountain ring; also, they might have peeked in the gap and decided there was nothing there and come back out. CIHN