Again, you want a secondary that can mix and match their various strengths and weaknesses of your DBs to give you the best potential for matching up against the various personnel you have to face on a week to week basis.
Gaines is a hip pocket cover DB. He is a + guy who moves fluidly, has good speed and quickness to react and recover when facing double moves and deep routes without getting torched. He's not a round #1 and wasn't drafted as such. Plus, given the level of competition he faced in college, anyone being realistic about his play should understand that he's going to have to be groomed and accept the learning curve it's going to take in getting playing time and experience under his belt. He's not a day 1 polished product, but the results so far are promising, and the assets he brings to the table directly compliment having a cover guy like Peters to pair with him and the rest of the secondary as a unit.
Here's a pretty good discussion of Peters, his strengths and weaknesses, and some fair criticism of how Sutton is using him- or at least putting him in spots where he's not schemed to be as effective as he could be?
The film room: Peters vs. Cooper
https://youtu.be/yaBuv2_oGdo