While there is definitely should be a fear of punishment, which is the lowest level of prevention of sin, the fear of Gehinnom is a fear of the SHAME which one will undergo when forced to face the sins they did. The punishment of Gehinnom is a SPIRITUAL one, since by that point, one’s neshama has left their physical body – and it is the neshama that undergoes the painful (for lack of better word) cleansing process.
Concepts such as fires, oil in gehinnom are used to convey the process to us in our limited level of understanding. The process is solely a spiritual one – and definitely much more painful than any physical one could be….
I hope this is clear, and feel free to correct/critique….