I don't think Goretzka would be a good fit, not as part of the starting XI atleast. He's an all-round player but doesn't posses exceptional abilities in any one facet of the game, a typical old fashioned Box-to-Box player; who have gone out of style lately. Pogba right now is our most important player and anyone we buy/sell in midfield should be made to get the best out of him and I don't think Goretzka is the right player for that. I think he can replace Herrera as a squad player if he does come.
Pogba's weakness right now for me is his ability to control the game, especially against the better teams in Europe and internationally. He has all the skill and creativity in the world but being able to control a game is a separate ability, which I don't think he possess as of yet. Therefore I feel we need to sign someone with who can control that midfield and allow Pogba to use that creativity and skill upfront, you can compare it to having an Alonso to allow Pogba to become a Gerrard (in terms of tactics).
Staying within Germany, I'd look to someone like Weigl who can fulfill that role. Maybe he's a bit too defensive at the moment but his ball retention and distributing abilities were ever improving under Tuchel (Bosz has left him much too exposed defensively this season). Having a midfield of Weigl-Matic-Pogba would be fantastic for the bigger matches, it'l provide control and creativity; for matches against lesser opponents we can opt for either of Weigl/Matic and play with a CAM.
All in all, given he is free and hugely talented, it'd be a very risk free bet but it wouldn't completely solve our problems. We'd still need another midfielder with the attributes of a peak Carrick to get the best out of Pogba. Goretzka would be great for Bayern though, slot right into the Vidal role.