My understanding is that g force is apparent weight/normal Earth weight, and is calculated by a+g/g where g=9.8. Now, apparent weight is the total force exerted on an object by its support, which is why a freefalling object on Earth (in a vacuum) experiences 0 g- there is no reaction force.
But let's say I am falling on some planet where acceleration due to gravity is 20ms-2 down. Will I still feel weightless? Why does g force formula give -1 g then?
Have a read through the answers to How can you accelerate without moving? and If $F=ma$, how can we experience both gravity and a normal force even though we are not accelerating? as these explain in some details exactly what is meant by acceleration in relativity.
If you are falling freely then by definition you are weightless i.e. your proper acceleration is zero (see the above links for what proper acceleration is). It doesn't matter whether you are falling towards Earth, towards your planet with a $20$m/sec$^2$ surface gravity or towards a humongous black hole. In all cases falling freely means you are weightless.
If you are not falling freely then some force is being applied to you, and as a result you experience an acceleration. So for example if you are stationary on the surface of a planet with surface gravity $g$ the force being applied to you (by the ground you stand on) is $mg$. The value of $g$ is $9.81$m/sec$^2$ on Earth or $20$m/sec$^2$ on the other planet.
If in addition you're in an elevator that has an acceleration $a$ then your total acceleration will be $g+a$ and the total force on you will be $m(g+a)$, and this equation applies to both your planets or indeed any planet.
