OP was asking about upgrading with miles, which officially cannot be done at the gate. Moreover, since this appears to be a domestic flight, it would be a pay with miles type upgrade to pay the difference to upfare the ticket, and doing this in advance is more advantageous. Finally, OP is GM, so if there were FC seats available for upgrades at the gate, he/she would be entitled to a free upgrade, possibly plus the companion (if companion had a DL FF number on the reservation, etc.)