It's not just about the look, it's about the feel.

From looking at the "gallery app" on android:

Flutter's input-text has their own custom mark text handler - no double tap, long press opens a custom menu that is missing native options (Such as translate text).

Changing the font size in the OS does nothing in the app.

Clicking on buttons doesn't make a tap sound if the "sound on tap" option is set.

I'm guessing no accessibility support - or very poor one.

There's probably many more tiny issues that customer's will complain about when you a product is shipped to production that will make it clear that this isn't a native app.