I dont use the gatling enough to say definitively; however, ink recovery is better for spamming subs when you are able to dive into ink. Meaning if all you were doing is spamming suction bombs while in ink, ink rec. would be superior. How ink saver sub works is it refunds around 10% of the ink (per main ability) you used for the sub weapon AFTER you use the sub, so 5-7% of your ink tank per sub use usually. It does not decrease the cost of your sub. This means that if you are using your sub at minimum ink, then youll have the ability to shoot a little afterwards instead of being empty. Its better for pinch situations in which your ink tank is not full. That said, with the gatling I've never needed to use two fully charged, full duration blasts in a row. Saver sub allows you to set a splash wall, charge a shot, let ALL your bullets out without diving, and repeat once more. When you dive, you regain the ink of those bullets you didnt shoot out, so I dont think the integer game here will ever come into play.
Even for sets you might want ink sub, 1 ink saver main or 3 subs is all you ever need. I never recommend stacking it for any set. 1 main is enough for those oh shit moments where you spam your sub weapon and then try to shoot and ur out of ink. It can be good for the other splash wall weapons, the custom, the 96 etc, but the gatling has more than enough ink efficiency to kill even an entire team after placing a splash wall without any perks.