Not sure about buying them at the gate, but you can buy them online here and pick them up at the ticket window. That's how I bought my five day park hopper, and I just had to show them my ID. I think you also have to prove that you live in SoCal.
Yes, the "Pay for Disneyland, get DCA for FREE!" ticket, and also the 2 day ParkHopper for the Price of one Day SoCal ticket specials can be purchased at the Main Ticket Windows. The Ticket Seller CM will ask for proof of living in the proper zip code area at the time of purchase.