Could you please elaborate how is your Gallery currently looking and how do you want to change to help you more. If you want to change the whole layout then we have a property called Layout. If that's not what you are looking for, then please elaborate to answer you.
If this post helps, then please consider Accept it as the solution to help the other members find it more quickly.
Thanks for your comment... It made me realize that I explored this option. But in my example I can only change it to one of the 3 Carousel options (Blank, Title and subtitle, Title and subtitle on overlay). If I change it to Blank, the elements in the Gallery disapear. My aim was to change a gallery with a vertical layout to a horizontal one. But this is only seems to be an option when I create a new Gallery.
Using PowerApps for some weeks now I am still struggling with the conclusion if an impediment is a lack in my knowledge or (still) missing functionality in PowerApps.