The feature is introduced with Gallery 12.1.07.5. It's same as the Samsung Internet feature, hide status bar. When you scroll down to see your photos on Gallery, the status bar will be hided.
Here is an example, see on the top of the screenshot, there is no status bar.
(Windows 11 wallpapers)
To enable it, be sure you have the 12.1.07.5 version. If you already have it, tap at 3 dots in the end, go to settings, and you will see there "Full Screen Scrolling". Enable it, and try if it works!
The version is available on Galaxy Store, download it now.
(Not for OneUI Core)
But this option does have no effect when you are scrolling to one image to the next (or previous one).
Now I got what it does. It only works when you are in the starting page, with the list of all the photos (or in Album or in Stories).
Not such a big change actually (IMHO), and the bar continuously pop up (hidying a part of the screen you were previously able to see) each time you start to scroll. It may be more confusing that useful...