Firstly, you have to toggle the Greg option in the debug menu. To do that, open the game settings, go to settings, click the burger on the top left part of the menu a couple of times until a bug appears on the right. Close settings, and open the debug menu. Scroll down until you see the Greg option and toggle it.
While holding shift, spawn at least 2000 people. Make sure you spawn them in groups, because for Greg to spawn you also need at least 3 cities.
Other disasters have to be enabled in World Laws.
For him to spawn quicker, change the game speed to the max by holding shift and scrolling up. It's also recommended to enable sonic speed in the debug menu. It's at the top.
You should change the age to Age of Despair so other disasters happen quicker. It's best to disable hunger, dying of old age, and wars so you can be sure the population is always over 2000. Then just wait. When he spawns, you will get the achievement.
Credit:
https://the-official-worldbox-wiki.fandom.com/wiki/Greg