I wouldn't consider 21 GPA high but I would consider 10 GPA low. Benefit of higher GPA is more coverage and better results. Lower GPA means more acres covered per tank which if your a big farmer and time is critical then you try to run on the low side but still enough to get a good kill. Some herbicides are systemic (2,4-d is systemic). That means you don't need as good as coverage as you would with a contact herbicide like Liberty or Paraquat because the chemical translocate through the plant to kill it.