I also remember Omaha having Mueller's gibbon, and that is what the animal in this photo appears to be.
Last I checked the only North American facilities with Silvery are the Gibbon Conservation Center and zoos in Fort Wayne and Greensboro; not sure who all has Agile.
As always when I have a primate question, I deferred to @ShonenJake13 who identified this as a Mueller's. Funny, I though about that species when uploading the photo but didn't think Omaha kept them!