Player dmg and GDT are 1:1 and subtracted as such: Player dmg % - GDT = final effective Player dmg %. (do a comparison between 0 GDT and 75% gdt with 200% player dmg which is very possible in pvp, you get only 25% reduced damage if you go from 0 to 75.
Player damage doesn't affect DT in pvp
If you have 100k damage, 0% player damage against 75% DT, you do 25k.
If you have 100k damage, 200% player damage against 75% DT, you do 75k (and not 225k)