The question asks for sources.
The Ramban wrote a sefer called Toras Hadam and in it he has a chapter (called “shar”, each part is another gate) named Shar Hagimul. In it he goes into great detail about the subjects of gehenom and gan eden and much more.
He discusses the judgment of Rosh Hashanah, The great day of judgment, the ways of reward and punishment, afflictions of love, Yisurim, Nisayon, Tzadik vra lo, Rasha vtov lo, Eyov and his friends debate, THE ESSENSE OF HELL, the punishment of Kares, THE REWARD OF GAN EDEN AND ITS TIME (when is it now or later…), the times of Moshiach and Olam Habah.
Warning: parts of it are scary. It also must be studied properly not just read through like a novel. There are a number of commentaries on it which can help. You can pick up a copy at many Judaica stores. I purchased one for 5.50 (barely any commentary though)
Good luck!