You didn't say what version of Gantry? or what template? But in both cases the blocks are doing what they should in mobile viewport.... LTR columns become a 100% width single column.
Except that neither blocks or grid content still doesn't support WYSIWYG editor, not even simple one, with basic formatting: bold, text align, image, url, bullets, which makes editing text a hassle.
Many users already asked for that since Gantry 5 deployment!
Sure, we can use other html editor or write code directly, but it takes extra time and effort, and it's certanly not non-techy user friendly, where clients needs to edit or add new multi column articles.
The following users have thanked you: charleyhankins