Gehinom is realizing the damage caused by your aveiros (in terms of your neshama’s kesher to Hashem as well as the damage caused to all of the oilamos through your aveiros).
Gan Eden is realizing the greatness of what you accomplished with your Mitzvos and the Light and Ruchnius that was brought to the world through your Mitzvos.
Of course, in this world we have no idea what that means and it doesn’t mean what we think it means, because we have no way of conceiving of it.
One important thing to realize though is that even though we want to try to avoid going to gehenom, if chas veshalom, lo aleinu, anyone has to go, gehenom is also for his good. Everything Hashem does to us is for our good, even gehenom, since it is for the purpose of cleansing our Neshamos so they can go to Gan Eden and have a kesher with Hashem.