The <gallery> tag has only one column in portrait orientation on mobile website. Landscape orientation has (fixed?) 4 columns. I think two fixed columns in portrait mode would be nice for traditional gallery mode. For "packed" mode images could be freely floating in portrait mode (multiple on row).

Other approach would be to rework the whole gallery for mobile view for it to be truly responsive. This would be the best case.

Mobile apps (Android tested only) - in portrait mode gallery produces has huge images (almost 100% screen width). Fix this the same way as mobile website. I think iOS app has the same problem (testing needed).

Screens for mobile website (Android):

Files - mode = traditional

Files - mode = packed