G-force is measured in a perception of weight. Positive Gs give you a feeling of additional weight, or in the case of lateral Gs, a feeling of literally being pushed to the side. Negative Gs produce a feeling of weightlessness. Mission: Space produces both positive and negative Gs; Space Mountain produces neither. It's just low velocity inertia because of the unbanked turns. Your body wants to keep going in the same direction, so you get thrown, but you aren't feeling any additional forces besides the sudden change in direction, therefore it's considered a forceless coaster.
It can also be bumpy. It depends on the vehicle, the wheels especially, since that is what is interacting with the track.
Sorry, you're definitely not using those terms as they normally are used.
A G is an accelerating force measuring the apparent gravitational force of Earth generally measured at 9.8 m/s^2. As such, it is also a vector, it is pointing toward the center of the Earth.
When one accelerates or changes direction going up and down along this vector, it is convenient to measure the resultant accelerating force in terms of G... as a handy form of standard measurement. If you're going down an incline at 5.9 m/s^2, then your resultant 'feel' of gravitational acceleration is 1/2G. If you're being propelled upward at 9.8 m/s^2, then the resultant feel of acceleration is 2G. If you're free-falling, it's 0G.
You acknowledge that Mission:Space produces both positive and negative Gs. No, it technically doesn't, since it is all motion perpendicular to the vector of Earth's apparent gravitational force. However, we can talk about the centrifugal/centripetal force of angular acceleration in terms of G in an analogous way. If the angular force is producing a lateral force vector of 19.6 m/s^2, then we can say it's producing 2Gs of force, even if the vector of the force is not pointing toward the center of the Earth, but perpendicular to the vector of Earth's gravity.
IOW, lateral forces can be measured in terms of G in an analogous way, namely, how proportional that force is to a force measured as 9.8 m/s^2.
So, Space Mountain. When you whip around a turn that isn't banked to give the resultant vector a downward direction in relation to your body, then you feel the whip around as a lateral force throwing you left and right and into the sides of car. All forces can be expressed in their absolute value to their proportion to the absolute value of G. Thus, one can express that lateral force as "Gs", just like you did for Mission:Space. It is literally (in the literal sense of literal) the same thing.
We've had a thread discussing the lateral G forces of various spinners and no one said, "That's not a G force, that's just inertia!" We all knew what we meant by "Gs". A sharp turn on coaster without banking is the same lateral force. And it can be expressed in terms of G. As most people do.
Except I wasn't sharing an opinion. You can call a tomato whatever you like, but there's a distinct difference between a ride simply having G-forces and being rough. G-forces can lead to roughness, but they're not synonymous. You're going to give people the wrong impression by pretending they're the same.