Also reproduced on Chromebook Pixel 58.0.3027.0.
I confirmed a touch event received here always have only 1 element. (which should have 2 while doing pinch action.)
https://cs.chromium.org/chromium/src/ui/file_manager/gallery/js/slide_mode.js?l=2021
I also saw the browser also have this issue.
By these reasons I think this is not a bug of the Gallery app.

For the content see regression  bug 698558 . Not sure if this is the same bug.

I think this should have been fixed at ToT now, because 695905 was fixed.
https://bugs.chromium.org/p/chromium/issues/detail?id=695905

I confirmed this is fixed on 59.0.3032.0.
We'd still need to make sure the change is merged to M58 release branch.

Change was merged into M58

9334.23.0 / 58.0.3029.39