While I will admit that a no-gi guy in a gi competition has more ways to screw up, it's nowhere near as absolute as you make it sound. A gi guy that hasn't trained no-gi is very likely to end up as meat (reverse is true as well).
If you haven't trained gi there's a few things that you can do. Obviously these will be more successful against white belts (it's rare to find a white with a good lapel choke game; they're usually blue by the time they get that worked out):
1) don't take grips...hold your opponent just like you're in no-gi
2) don't attempt lapel chokes
3) high pressure, high pace
4) escape via position not attempting to slip