Asking a Phyics question for my brother.?

"If the force of gravity gets smaller" You might mean the force, which is dependent on the distance between masses, and the size of the masses. The force of gravity of the earth on a feather is less than on a large lead weight. And the force on an identical weight would be different if the weight were a great distance from the earth. Or you could mean if the gravitational constant G were to change

"to the extent of infinity". Perhaps you mean infintite distance, which would reduce the force, asmptotically to zero, or perhaps something else.

"does it get smaller than the planck life length?" Planck life length isn't a thing. Planck length is (it is very small. But you can't compare length to force, so that doesn't mean anything. There is also a notion of Planck force. It is very big (10^44 Newtons) so I don't think that you mean anything by that. In planck units, the gravitational constant is by definition equal to 1.

no