Uploaded images are 800 x 800 px.

Need this code in Divi > theme options > custom css:

/*image gallery item aspect ratio square 1:1*/

.pa-image-gallery-1-1 .et_pb_gallery_image {
padding-top: 100%;
display: block;
}

.pa-image-gallery-1-1 .et_pb_gallery_item img {
position: absolute;
height: 100%;
width: 100%;
top: 0;
left: 0;
right: 0;
bottom: 0;
object-fit: cover;
}

And this in Gallery>Advances>css:

pa-image-gallery-1-1