I think that's what we're going to do tomorrow, review the iApp. One line that keeps sticking out to me, that may or may not be the issue, is this one, found on page 21 of the PDF:

In the Virtual Servers and Pools section, complete the following. a. Type the IP address for the virtual server. b. Type the FQDN to which external clients will connect with the Horizon Client.

Item B. in particular. Right now our F5 admin has the DC2 LTM FQDN in there for the DC2 iApp, instead of the GTM. The users will actually use the GTM to connect, but, I guess he went with the LTM FQDN upon creation. He did the same on the DC1 iApp. For that entry, he has the DC1 LTM FQDN, instead of the GTM. I can see where he's going with that thought process, but, might have him switch to the GTM and test.