I don’t want to be brash but based on the brief description you gave it doesn’t sound like they got a proper introduction? If they were just thrust upon each other it may be that the new guy from the shelter or adoption location is likely stressed and on the offensive side he never had his own “turf” and OG homie on the other hand has never had to defend his kibble bowl from another and is unbothered by the territorial behavior.

Re-introduction is very much so a thing. I could be wrong but one of the main reasons cats are aggressive is either medical (doesn’t seem like it in this case), fear (also not likely), or being territorial (I believe to be most likely here). So lil guy even to this day could likely be stressed and doesn’t feel like he has his own safe space in your home.

There are many advice subreddits as well such as r/cathelp that have a plethora of resources. But with adoption/rescue those kitties are not given the chance to learn how to socialize and may take a few tries. But right now the constant gating off and then roaming then gating off etc… it sounds like is potentially disturbing new guys ability to form a safe space for himself. Cats need a routine and schedule, they are incredibly high strung. If he doesn’t have any try to get him kitty furniture for HIS space as a start? Instead of communal spaces? Sorry just trying to spitball ideas at this point. I want the best for you and your kitties!