The Earth pulls on objects near its surface with a force we call one G. However, if you were to step onto the surface of another planet, the gravitational pull you would experience would most likely be greater or less than one G. An astronaut walking on the surface of the Moon, for example feels a gravitational pull of only 1/6 Gs. This means that if he weighs 150 pounds on Earth, he will weigh only about 25 pounds on the Moon because the gravity there is only 1/6th as strong! However, if this same astronaut could stand on the surface of Jupiter where there are about 2.6 Gs of gravity, he would weigh in at a whopping 390 pounds.
To determine how much the G force is on any given planet, you must know some things about the planet. First, what is its mass? The greater a planet's mass is, the stronger its gravitational pull will be. However, mass will not tell you everything. For example, the Earth has 80 times more mass than the Moon but only about 6 times its gravitational pull. The reason for this is that a planet's pull acts as if it starts at its center, not at its surface. And, because gravity decreases exponentially over distance, a planet's surface gravity will be weaker, the farther the distance is to its center.
For more on Newton's law of universal gravitation see What is Gravity Page 4.
To see a Java applet that calculates weights on the surfaces of different planets click here (requires a Java enabled browser).
