What are Arduino digital pin number(s) that are assigned to
the Gator's Serial Port 0 (default USB port) PD0 & PD1 pins?
Since those are dedicated serial port pins they do not have Arduino digital pin numbers. You can always add them yourself by editing the hardware/cores/arduino/pins_arduino.c file.
I know that by soldering wires to these pins voids
my warranty on the Gator?
Yes, those lines are not externally protected thus if you apply overvoltage or negative voltage from an outside wire then you run the risk of damaging the AVR or the FT232 device.
BTW ... Soldering two wires to the surface mount 324P
is not an easy task for those attemping to try it!
It is probably easier to solder some thin wire (e.g., 30AWG) into one of the vias for those lines. This image shows two accessible locations for soldering to the TXD and RXD lines: