What causes the rider on a roller coaster to experience the + or - g's if gravity always remains the same on earth

Yes both = and - g can be felt by a rider in a roller coaster.

Explanation:

It is crucial to understand how we feel gravity in this case.

We humans have no sensory organs to directly detect magnitude and direction like some birds and other creatures, but then how do we we feel gravity?

When we stand on our feet we feel our weight due to the normal reaction of floor on our feet trying to keep us stand and our weight trying to crush us down. In an elevator we feel difference in our weight (difference magnitudes of gravity) but actually we are feeling the differences in normal reactions under different accelerations of the elevator.

In the case of roller coaster you will feel + g as you sit on a chair in it, but will feel - g when you are in upside down position as roller coaster move.

When you are seated you will feel the normal reaction of seat on you giving you the feeling + g and the support of the buckles to stay in the roller coaster when you are upside down will give you the - g feeling.

This is just the physics approach, a biological approach can be given in association with sensors relating to ears.