Personally I despise rubber tires. They eventually fail, requiring periodic replacement. They limit electrical pickup by reducing the number of conductive wheels, and as they disintegrate they leave black residue all over the track.
I'm not sure what the product number of your GP-9 is, or what vintage. My first recommendation would be to add a 2nd motor / powered truck. This could be easy or difficult, depending on whether your existing powered truck also has collectors (pickup rollers), and how it is mounted to the chassis. This will greatly increase tractive effort because now 100% of the loco's weight is on driven axles. THEN add more weight, as low as possible in the fuel tank or chassis.
It's never good to add so much weight that the traction exceeds motor torque. This is another reason I don't like rubber tires- running the loco above the motor's current limit, especially at slow speed, can burn out motors and circuit boards. Some books will tell you to add weight to the point where the wheels just barely slip at the motors' full rated current. Personally I wouldn't add more than about 10 ounces.
If your loco can't pull the desired train, look at each car and make sure the wheels rotate freely. If not, put ONE drop of light oil on each car's wheel bearing. Make sure the loco wheels AND your track are clean and free of oil. Also make sure the loco's wheels are centered on the axle, and not out of round due to a manufacturing defect.
If your loco STILL can't pull the train, you will have to reduce your slope / grade, or do as the real RRs do, and add a 2nd powered unit. With command control, a helper operation could be very interesting! Hope this helps!
