Source: https://www.elegantthemes.com/forum/viewtopic.php?f=187&t=573199

A Divi gallery with 4 images in a row.

• On iPad portrait, it shows a row with 3 and a row with 1 image rather then 2 / 2
• On iPhone 6 Plus, there is a border on the right

/* -- START Fix responsive issues for gallery with 4 images at the bottom of each page -- */
@media (max-width: 980px) and (min-width: 479px) {
.et_pb_gallery .et_pb_grid_item:nth-child(n) {
width: 50% !important;
clear:none;
}
.et_pb_gallery .et_pb_grid_item:nth-child(n) img {
width:100%;
}
.et_pb_gallery .et_pb_grid_item:nth-child(2n+1) img {
clear:both !important;
}}
.et_pb_gallery .et_pb_grid_item:nth-child(n) img {
width:100%;
}
/* -- END Fix responsive issues for gallery with 4 images at the bottom of each page -- */Code language: CSS (css)

To show other/multiple columns on different breakpoints:

• https://www.elegantthemes.com/blog/divi-resources/changing-the-number-of-columns-in-the-divi-gallery-module-at-different-breakpoints