Question

What will keep the gravitational force between two objects unchanged?

Answer 1

The gravitational force between two objects is determined by

-- the product of their masses, and

-- the distance between their centers.

If you want to keep the gravitational force between two objects unchanged, then you can make changes to the mass of one object, the mass of the other object, and the distance between them, BUT you have to be very careful to always keep the quantity

(one mass) x (the other mass) / (the distance)²

the same, unchanged number.  This means that if you change one thing, you'll have to change something else to keep that expression constant.

For example:  If one of the masses suddenly somehow becomes 4 times as great, then you can EITHER change the other mass to 1/4 of its original mass, OR move the two objects twice as far apart.