The point of GaaS is to use a carrot and stick method to keep players playing for as long as possible. This is done by continually introducing new gear that is stupidly hard to get, so that players are forced to grind forever for said new gear. Splatoon doesn't really have this though. New weapons aren't that hard to unlock, and aren't completely overpowered compared to older weapons.
Now compare this to games like the Division 2 where there's a Sniper Rifle so OP that it makes being the Sniper Class completely pointless. Oh, and to get said OP rifle, you need to do several different raids that are only available at random for a week's time. Or something like that. I forget. My boss was telling me about it the other day.
Anyway as Rol said above, Splatoon 2 is not GaaS. It doesn't have extreme grind, power creep, or lootboxes.
