I do most tuning on the dyno in 4th - high load and cruising. But there's no reason why you can't use other gears. 4th is used more to ensure consistency than anything. 4th also tends to be 1:1.
1st and 2nd are no good though cos everything happens too fast and most cars will have traction problems anyway.
I usually think of "low load" as the area you'd use while the car is cruising at low throttle opening. So in the maps you'd be looking at the first 2 or 3 load columns.