that’s not an issue with the Gallery. That is an issue with your images.
Most likely one of the following is happening.
1) the original image is smaller than the viewing window so Divi is enlarging it.
2) the original image is too large and WordPress has reduced its size in an effort for it to load faster.
I aim to have JPG images that are no larger than 2000 pixels wide or tall and have a size under 500kb (200 kb is even better)