I would say it depends on how the regulation is worded. The gate guard is likely full of it, and should have been challenged by requesting his supervisor come to the gate. I can't imagine the regulation not having an exception for service animals. If it doesn't, it probably runs afoul of the ADA, but that would require a court challenge to establish.
I would just question the use of a pit bull for a service animal. Justified or not (I think it is), pit bulls have a rep for being particularly dangerous. While I would not challenge the right of someone to have a pit bull for a service animal, I think it is unwise, simply because of how others would react and how easy it would be to get an animal that would be just as serviceable, but without the rep.
(I hate having to put the disclaimer, "While I would not challenge the right..." in this post, it has become necessary due to the prevalence around here of late of assuming that advising against amounts to denying rights.)