I don't really know Gerwig, either. Looked her up. She's done some acting, but now she mostly seems to be about writing/directing lady-centric stuff. So this Barbie will probably have some strong depictions of female empowerment, etc. at its core.
And I'm not saying that to be condescending or cynical or whatever, I just believe that's what this movie is going to be about.
Because like I said, if you're making a Barbie movie NOW, it probably ain't to sell toys. Although the toys will undoubtedly see a boost in sales, I'm sure.