Parshas Bireishis: When And Where Were Kayin & Hevel Born?
The Otzar HaPlaos brings from the Zohar Chadash Medrash Rus that Kayin and Hevel were both born after Hashem chased Adam out of Gan Eden.
However, the Otzar HaPlaos also brings another opinion that Kayin was born in Gan Eden and Hevel out of Gan Eden. Kayin was called Kayin, "Kanisi Ish Es Hashem." Since they were in Gan Eden and were destined to live forever, Chava named her son, Kayin, which means an eternal Kinyan. Hevel was born outside of Gan Eden, after death was already decreed on man, therefore, he was called Hevel or folly.
The Gemara in Sanhedrin (38b) seems to indicate that both Kayin and Hevel were born in Gan Eden before the Chait. The Gemara says that they were both born in the eighth hour, Hashem commanded not to eat from the Eitz HaDaas in the ninth hour, and Adam and Chava ate in the tenth hour.