The Goliath is described as the "Ultimate Horizon Mexico event" in Forza Horizon 5, and it's hard to disagree when it consists of a huge 55KM sized circuit lap around the entire map. It takes around ten minutes to complete, but rewards 40,000 XP, and unlocks the "Long Gone" Achievement for winning the event.
It is not available from the start, and players will have to put in a little bit of work to unlock it. However, it shouldn't take too long to get, meaning players can unlock the Goliath
Read more on gamerant.com