Nah, there's no real required reading. The Settlement 2 thing was mentioned in passing in Gears 4, and elaborated on in the collectibles IIRC. Long run wasn't a big deal, it's just a story prop used to demonstrate the COG hasn't changed at all once it was rebuilt. It's still systematically obsessed with controlling its population and ordering their lives to the letter. Some people in Settlement 2 didn't take very kindly to this when they started clenching the iron fist.
Jinn's one line about in Gears 4 says really all you need to know about how the COG views the situation: "A highly successful operation. It's now our most compliant settlement."
The big conflict rift between JD and his dad in 4 was going against his wishes and joining the new CoG. Marcus didn't want anything to do with the CoG, and he always had a problem with how the old one dealt with society to begin with.
Aaaaaanyways, Finished with Gears 5. Hoooo boy, what a ride. This thing goes crazy off the rails after Act 2.
And then at the ending
Actual EDIT (instead of all the little microedits I did): I do love the insane number of callbacks to pretty much every nook and cranny of the Gears lore that exists either in the story itself or through the collectibles.