For what its worth, I bought a walk-up ticket at the gate on UA before in FLL after missing an AA flight. This is when they had gates right across from each other (I think they are in different terminals now). The gate agent didn't seem phased and seemed happy to sell me the ticket; she just asked that I wait until the boarding process was over so she wasn't distracted, took 3 minutes to sell me the walk-up fare, boarded me, closed out the flight and closed the door. Granted, I was PassPlus Assured at the time, so that may have helped motivate her, but I didn't ask her to mess around with issuing a PASSA ticket as that takes a while (and was more expensive than the walk-up fare anyway), so I just put it on a credit card. Was easy and done.
Those who say gates/airports are no place to buy tickets are just 'plane' wrong. It is fine as long as the agent has 3 minutes to spare.