I am using this driver to operate 3 sparkfun 6.5" 7 seg displays
My question is "if I have a current limiting resistor, do I need to mess with the special Programmable Global Current Gain mode" ?
Simple answer is yes.
It is a constant current driver so you need to set it. If you have a resistor it will make no difference to you having non. The output will adjust so the current is what you set.
DO NOT put a resistor in series with your LEDS with that chip.
That chip is special. You use ONE resistor on the chip to control the current through each ON LED.