Play calls are something entirely different. They're also a smaller deal than "the whole offense's structure."
So if we ever hear that Gattis is accepting play call inputs from others including Harbaugh, my response will be "oh that sounds normal - good."
Collaboration is good. And it's wrong to think Harbaugh wouldn't have loads of objectively helpful ideas for any OC. That doesn't even have to be Harbaugh braggy. Many HCs out there, perhaps a majority, have brilliant things to tell their OC.
By hoping Harbaugh goes mute on playcalls, I guess I'd say you're being rigid and setting yourself up for disappointment ... even if the offense is fully new and amazing. In 2019, it's inevitable that there will be moments where some sort of Harbaughian play is better than what Gattis had considered.
And he can do that without taking one inch of the offense back from Gattis.