When creating a new gallery in 3.5, it displays the thumbnails of the images selected along with "Gallery Settings" on the right. When changing the option for "Columns", the gallery preview (not sure if it's meant to be a preview, but it does feel like one) does not update to reflect the selection.
Example: create a gallery with 4 images. The default column setting is 3. Change that to 2 columns. I, and likely others, would expect the "preview" of the gallery to show 2 columns of images, but it doesn't change the display. See screenshot. The display seems to adjust according to resolution (responsive), which is great. In the screenshot above, if I widen the browser it shows 4 wide, and if I shrink it it'll drop to 2 wide.
Thinking about it as a common user, and as a user who would want to preview what my new gallery is going to look like, I think it makes sense to expect the gallery images to be displayed in the number of columns selected in the Gallery Settings.