In my experience, higher GP always nets you the win - if you both have an equally good strategy.
Case in point: My last match was against a guy who had Darth Revan, Revan, Padme etc. I took a break for 3 months, so I have none of these characters. However, I still won. Why? Because he kept these teams for his offense, which netted him 3 wins, then he died to my Nightsisters and CLS. On the other side, I was able to clear his entire defense which I wouldn‘t have been able to do if he set his teams on defense.
Point is: Going against a guy with a better roster is definitely tough, but not an auto-lose. GP is definitely important, but cannot fully compensate for bad strategy.