I'm working on a diplomacy map and I want to make it so when their morale is low one of their random cities (Gateways) has a rebellion thus turning it white. How do I achieve this?

just have a white computer player and if their morale is low give one gateway to that computer player.

Use the custom score to track the morale. When it falls below 11 (max 100) give 1 Warpgate to player 7 (white).

Human Players
Conditions:
Current player Custom Score is at most 10
Actions:
Center Location 'Morale' on Warpgate owned by Current player at anywhere
Give all [any unit] at 'Morale' to player 7
Add 10 to Custom score for Current player
Preserve

This will give the leftmost Warpgate to white and add some morale, so the next rebellion won't occur directly after.
If you want a random Warpgate to be given instead of the leftmost it would become quite complicated...

A simple way to do it would be to assign every city in the game a number and have a computer player's death counter constantly having 1 added to it with hyper triggers (looping back to 0 once it hits how many # cities there are). Once moral is low enough, just a trigger like this for every city:
Current player brings atleast 1 building to (#23), morale is atmost 10, p8 has suffered 23 deaths: give (city) to (white player), add morale, preserve trigger.
That way if you don't own the city # it's currently on the counter will just keep adding until it gets to one you own.

This would very likely be the easiest way to do this. If you still have any questions, post away