How can I change the aspect ratio for Gallery in Mobile using CSS?

How can I change the aspect ratio for Gallery in Mobile using CSS? It's currently set to 1:1 on Grid: Simple. This is the code I am using to control the gallery at the moment:

@media screen and (max-width:800px) {
.gallery-grid-wrapper {
    grid-template-columns: 
  repeat(1,1fr) !important;
}
}

What can I add to the code to change the images within gallery to either 3:4 or 2:3 AND where in the code do I add it?

Image attached showing what the site looks like in mobile

What is gallery page url? We can help easier