Yes, the force of gravity lessens very slightly as you move away from the surface. It is not linear. But gravity is not what is in question with a helium balloon floating. The rubber of the balloon and the mass of the helium is in fact attracted to the Earth, yet the pressure or density of the atmosphere is higher than the balloon filled with helium therefore it will float up through the atmosphere until it pops from low pressure, then the volume of the balloon is reduced but the mass of the rubber making up the balloon is the same. It is no longer less dense than the surrounding atmosphere so it has nothing to float on and it will fall due to gravity.
The atmosphere acts like a fluid, and it obeys fluid dynamics. The less dense something is will rise to the top, until it reaches a layer that is less dense than it. So it will not continue to float up and escape Earths gravity unless the atmosphere extended out forever at a density higher than the balloon, which it doesn't.
Another thing is, that air pressure is a result of gravity also, the closer you get to the surface of the Earth the higher the pressure or density of the air is. The same with water. The further down below the surface of the water you go the higher the pressure. The water above is pressing down on the water below, the same as the atmosphere.
I know JRowe thinks I am evading, but he is quite ignorant of many things so anything he doesn't understand is evasion in his mind. He is completely irrelevant so pay him no mind please. Also I do not work for the government modestman, I am not paid to give you information. I speak to what is true, what I have learned, seen with my own eyes, and/or verified myself through math or experience.