How does force of gravity and centripetal force work at the top of a loop?

There is a misconception here.

The centripetal force is the force needed to make an object move in a circular path - or put differently, for the object to accelerate in a direction perpendicular to its motion.

When, for example, you swing a ball on a string in a circle, you provide that force through the tension in the string.

A car on a rollercoaster feels a force from gravity, and another force from the rails. When you are at the top of the curve, these two forces push in the same direction - and this means you will either:

1. have less force from the rails (to the point where sometimes you are actually "hanging" from the rails - the wheels on a roller coaster grab from both the top and the bottom)
2. go around a much tighter loop (also because you are typically going slower, you need the loop to be even tighter).

People are sometimes scared of the "virtual twin" of the centripetal force: the fictitious centrifugal force (which is very real in a rotating frame of reference). If you need the car to be kept on the rails by its motion at the top, it is sufficient that the centrifugal force ($F_c$, force pushing outwards) is greater than the force of gravity ($F_g$, pointing inwards). The net result will be an inward force by the rails ($F_r$); added to the force of gravity, this will keep the car on the rails.