Here I go again lol, gravity don't push upward. It is opposing the force of buoyancy and therefore we can measure how strong the force of buoyancy is by using the gravity. That's why the force of buoyancy is equal to the WEIGHT (mass * gravity) of the fluid displaced. Gravity push downward and force of buoyancy push upward trying to resist the gravity action which is trying to pull the substance down the water. The more the substance is submerged in water, the stronger the force of gravity and therefore the more the fluid is displaced and therefore the more the force of buoyancy.
I'm not sure If I was able to explain well but I suck at teaching.
Example would be a simple brick submerged in water. Since gravity is constant, the more the brick is submerged in water, the more it displaces fluid and therefore the more the weight of fluid (M*g) is displaced). Gravity is constant but a more proper term is the force of gravity which is what is affecting the buoyancy force. The more the force of gravity (M*g) the more the buoyancy force). Just like newton's law; when you push hard on a desk, the desk pushes as hard back at your hand. The harder you push downward on the desk, the harder it push at you back and that's why the desk is not broken. So the more the force of gravity downward, the more the force of buoyancy upward.
