Yep, that is what the science books say, and it is based off an insane, rich guy who did experiments with huge (some say gold) balls in a barn. Never repeated.
And there is a lot of evidence that it is not the case. If gravity was based off of mass, then would the gravitational constant vary over the surface of the earth? I mean a mountain is tiny thing compared to the supposed diameter of the earth. The gravitational constant (big G, in the formula above) varies greatly from place to place and time to time.
I am unsure what do you mean by the gravitational constant varying?
Do you mean the speed at which you get accelerated?
If so, I haven't heard of any experiments that would prove it varies greatly place to place.
That is because science doesn't talk about it.
They have even gone so far as to FIX the speed of light, as it kept varying.
Okay, I guess I wasn't clear enough.
Can you provide any examples of such experiments that were done in a scientific way - meaing repeatable.
Try following this professor.
Rupert Sheldrake - The Science Delusion BANNED TED TALK