My understanding is no, you can't.
PCB123 along with ExpressPCB (there may be others) represents an effort by PCB manufacturers to sell "lock in" software which is free up front but only lets you order PCB's from that specific manufacturer.
Needless to say, I am not a fan of this approach
I would recommend you switch to Eagle, KiCad, or gEDA for which there is no lock-in and you are free to generate and use Gerber files as you see fit.
: compatible with Arduino UNO, 24V operation, all I/O's fused and protected