For what it is worth, in terms of potential for natural behaviour I think the zebra enclosure is worse. But in aesthetic terms you can't deny the gibbon enclosure is unattractive and offers poor viewing opportunities.
in the 1950-70's era Gibbons at London lived in a long & tall meshed enclosure alongside the Central Lawn (near where the animal display area is). There was a second similar enclosure holding Parrots and Cockatoos. The Gibbons were usually a breeding pair of Lars + their offspring. They had far more space(both length and height) than these White-Cheeked.
I don't think the Sobell cages were ever designed with Gibbons in mind. Somewhere along the line they have ended up in here though its far from ideal for them.
I think the only really good Monkey exhibit at London currently is the Mangabeys- that does work nicely..
