Of all the Gorilla enclosures I've seen in Uk and abroad, I would rate Paignton's (at least up to 2005) the hardest anywhere to see the Gorillas in their outdoor situation. My complaint is you couldn't SEE the animals even when they are moving about, NOT that they don't show hemselves. There's a big difference between the two. This is because the increasingly dense vegetation on the public's side of the water obscured views of the island & its occupants at all but a couple of places. The first time I ever saw the exhibit I was struck at how few viewing points situated around it there were. Then even they became overgrown though Gigit tells me some of the veg has been removed to give a better views now.
The more vegetation on the island itself the better and the gorillas can always retreat away from the public viewing areas if they want. But as you've noticed, they'd still rather prefer to stay indoors a lot where people can stare directly at them through glass. I'm not sure that Apes are really bothered at all by crowds of people anyway- so long as they can get away if they wish to. The Gorilla groups at Howletts are all happy to go about their business without trying to hide away from the almost permanent flow of visitors standing only a couple of feet outside their enclosures. Sometimes they go into the less visible areas too but nobody leaves Howletts complaining they can't see the Gorillas, and this is the most successful breeding programme in the World.