The surface gravity of a planet depends on two things, how massive the planet is, and how far away from the center the surface is. So ALL ELSE BEING EQUAL, if the planet is twice as heavy, its gravity is twice as much. If the planet is twice as big, the gravity is one-fourth as much. ( g @ m / r**2 ).

If the material is porous and fluffy, like a gas, the planet will be bigger and surface gravity will be less. If it is dense and compact, like stone or metal, the planet will be smaller and surface gravity is higher--assuming the total mass is identical in both cases.

Newton's laws allow us to "weigh" planetary systems because their measurable orbits are determined by their masses. But the motions are measured in different ways. Planetary gravity can cause a star to shift in position from side-to-side as its planet orbits, But this is motion across our line of sight. Motion in the line of sight (toward and away) can only be measured spectroscopically--Doppler shift.

Unfortunately, in the real world, what we are watching is a combination of the two, and the two motions must be unraveled from one another

For example, imagine a planet in a circular orbit around a star, and the plane of that orbit is tilted 45 degrees away from us. The star will move, tugged by the planets's gravity, from side to side (across line of sight), and towards and away from us (in line of site). THe former motion is detected by precisely measuring the star's position relative to surrounding stars. The latter is measured by noting the blue and red shift of the spectrum. It isn't always possible to do both, especially since they involve totally different observational strategies.