There are  a couple of reasons the Salford pulls easier. The gangs will make a machine pull harder a little but not a lot.

The greatest difference is the amount of steel going through the ground at once. On the Salford the blades are thinner because of the rock protection it has and the spacing is wider per blade. so if you would put all the blades in a stack for both machines the Salford stack will be much less than the GP. now you do the math which is easier to pull when engaged in the dirt.