Gravitational force (or G-force) is the force of attraction between all masses in the universe. Meaning everything in the universe have an attraction to one thing or another. Meaning, both forces are equal in magnitude and opposite in direction. Opposites attract. Newton's Law states that every object in the universe attracts every other object. Gravity is the weakest universal force meaning it is weaker than nuclear force.
An example of G-forces are with roller coasters. As you go up, the force is hitting against you. The Top Thrill Dragster below is one of the fastest and tallest coasters in the world.
Top Thrill Dragster