Use GP3Phisyc editor. You can find in gp3++ as part of export procedure to the game.
I think there is also an othe version more recently , more rich, where you can edit the brake balance too.
Anyway the grip in phisyc editor means in terms of, tyre grip (lateral particularly), downforce coefficient and drag ratio. If you increase the downforce coefficent it will have more tyre consumpion and more downforce per the same value of wings in setup, so you will need modify the setup too. Similar for drag ratio, it is how much air opposites to your forward at a known downforce.
mind the weight has got effect on the grip, cosy is the vertical force on the tyres. But more weight is actually more inertia also, so you shouldmodify the X - Y - Z rotational inertia moment.
and worse, it make you modify springs , ground height of the car, and pakers if u used in setup