With that said, let's explore the possibilities of Carcer City being in the GTA 6 map. ... For those wondering, this is the universe in which GTA 4 and 5 are set, and likely GTA 6 will be too.

In order to participate in GTA 5 Off-Road races, you will need to complete the mission “Mr. Philips”. Once you do so, you can participate in these races with any of the three characters.