Gravity
Gravity is still a mystery to us, yet we know and can quantify the effects of Gravity.
Currently 'Gravity' can be defined as mass warping spacetime; Einstein called this time dilation.
Note there is a difference to the 'Force of Gravity' (In Newtons) and what is know as 'big G' (Gravitational Constant)
gravitational constant G = 6.67408 × 10-11 m3 kg-1 s-2
This is as close as experimentation can get us so currently is ± 0.01%
SpaceTime
Newtonian Gravity
Sir Isaac Newton made a breakthrough in the 17 century. The strength of Gravity's effect is measured by a Force called Newtons in honour of Sir Isaac.
Isaac Newton published his comprehensive theory of gravity in 1687. Though others had thought about it before him, Newton was the first to create a theory that applied to all objects, large and small, using mathematics that was ahead of its time.
But Newton's equations just approximate the effects of Gravity without explaining how gravity worked.
Einstein's Gravity
Gravity is most accurately described by the general theory of relativity (proposed by Albert Einstein in 1915) which describes gravity not as a force, but as a consequence of the curvature of spacetime caused by the uneven distribution of mass.
Yet although Einstein's equations accurately defines the effects of Gravity on mass without explaining how mass works or why something like photons are 'massless'.
Equivalence principle
In the theory of general relativity, the equivalence principle is the equivalence of gravitational and inertial mass, and Albert Einstein's observation that the gravitational "force" as experienced locally while standing on a massive body (such as the Earth) is the same as the pseudo-force experienced by an observer in a non-inertial (accelerated) frame of reference.
Einstein deduced that free-fall is inertial motion. Objects in free-fall do not experience being accelerated downward (e.g. toward the earth or other massive body) but rather weightlessness and no acceleration.
How Gravity Makes Things Fall
Graviton
The graviton is a hypothetical particle which is thought to be responsible for carrying the force of gravity, in analogy to the photon, which is responsible for communicating all electromagnetic forces. Like a photon, it is a massless particle.
All current theories' of gravity imply that gravity effects extend forever outwards.
The misnomer of the term 'Zero G'
The commonly-used term “Zero-G” is a misnomer: gravity is always present in space.
A more correct term is 'free fall' given the perception of weightlessness.
When moving fast enough to achieve Orbital velocity an astronaut and any experiment orbiting the Earth in free fall experience weightlessness due to moving fast enough around the Earth; so for every meter you fall towards the Earth is the same amount the Earth curves away from you, causing you to never get any closer to the Earth. This also applies when travelling beyond Earth's Orbit you are in freefall around the Sun, likewise interstellar space beyond our solar system it is then in free fall around the Milky Way.
Vehicles like aircraft can experience apparent weightlessness briefly before they fall back to earth. (Faster vehicles can experience apparent weightlessness continuously, when they go into orbit around the planet.)
The misconception of the term ‘Zero G’ is very misleading, every part of our known universe is affected by gravity, there are relatively low null points that are called Lagrange points where multiple bodies gravity may cancel to some level. (maybe even to a level of a true Micro gravity environment)
Fact Box
Interestingly the ISS is still experiencing over 90% of Earth gravity yet astronauts in the ISS experience weightlessness due to the ISS being in Free Fall around the Earth.
G-force
Don’t confuse the term G-force or 'gforce' and G being defined as the acceleration due to effect of gravity at mean sea level due to the mass of the Earth being 9.81 meters per second squared(Newtons).
You can experience zero gforce in a fairground ride or as I have while training to learn to fly a light aircraft at the apex while doing a 'power on stall' where you can experience a second or so of weightlessness(zero gforce) similar to the training aircraft used by NASA.
Also SEE Artificial Gravity
Check out Constant 1G thrust you may well find interesting as well.