Regardless of whether Matip was a free transfer, he was and is a good player. Liverpool went out and bought Konate for big money because 2 good CBs isn't enough, and Gomez is often injured (and also not very good).
White, Gabriel and Saliba will push each other hard for the 2 spots. The competition will be brilliant for them and their development, and even better for Arsenal. I expect through a long season we'll see various combinations as their form and fitness varies, and I think the stuff about left and right side is way overplayed.