[Baron Munchausen]

Originally posted by Suicide Junkie:
Just multiply average density by volume.
kg/m^3 x m^3 = kg

Useful for rocky/icy planets, but will be trouble on gas giants due to the extreme changes in density going down to the core.

For surface gravity, F= GMm/r^2
Where gravitational constant G = 6.67300 × 10-11 m3 kg-1 s-2
Big and little masses M & m and the surface radius.
Little m will cancel out when you do F=ma to get the acceleration.
A = GM/R^2
Just remember to use the same units throughout.

PS:
Have fun picking a surface for your gas giant, and don't count any material above the surface in your mass calculation

Actually, don't forget the stuff overhead when figuring gravity. It has mass, too. The gravity inside a gas giant could be very weird. The atmospheric pressure is probably a more predictable problem.