With Gallo's lack of mobility and his height (he's probably the tallest player in the rotation) I'm betting he plays a bunch of center. Or, at least, a bunch of minutes where he's the nominal center.
Grant, on the other hand, showed he's capable of sticking with quick wings last year. I actually think we'll see a lot of Gallo and Grant together with Tatum (who's either still on the floor or has come back on after a short rest) where Gallo is the tallest of the 3, Grant is the strongest, and Tatum is the best rebounder and there's a lot of switching going on anyway.
Regardless, Grant is a decade younger than Gallo. He's not getting pushed out by someone that much older.