Gravitational force is derived from masses. The heavier the object, the stronger the gravitational force it exerts. All matter exerts gravitational force but we do not see their effect in our everyday life because they are very light.
Take bowling balls for example, they may be heavy to us but compared to the earth, it is nothing which is why we do not feel the bowling ball's gravitational force.
The earth is not the planet with the strongest gravitational force because it is not the heaviest planet in the universe.
Some small asteroids group together as they travel in space because of the gravitational force they exert on each other which is also why exploding a giant asteroid with a nuclear weapon will not make it go away. They will still be traveling in the same path but as a group of smaller asteroids which may deal the same or even more damage.