Game physics engines assume a certain scale. How you implement gravity is going to depend on the details of your project. If you are just dealing with one planet, check out https://docs.godotengine.org/en/4.0/classes/class_physicsserver3d.html and AREA_PARAM_GRAVITY_IS_POINT and AREA_PARAM_GRAVITY_POINT_UNIT_DISTANCE. If you planet even close to the real life earth size you might consider compiling the engine with double point, you would have to test it.
If you are modeling a solar system, you would set global gravity to 0. Use the _physics_process() for each object and calculate the forces exerted on the object (n-body problem if you want L points, or sphere of influence calculations) using F= G m1 m2 / d2