WP 5.3 I use Photo Gallery v 3.2.21, Fixed, Width=100%, Display only first image, my images are 700*700 px.
When I view the gallery on a mobile phone the images in the gallery are to big and not responding to that it’s a mobile screen.
According to the documentation this should work.
I solved it by choosing #6 in Popup box themes or I could pick one of the others but then I had to check Fit to screen. But in the other cases the borders disappeared on one side so I had to stick with #6.