Hello,
The Wix Pro Gallery and many other galleries are responsive so you shouldn’t have a problem viewing on smaller devices. If the gallery has a Stretch icon it is responsive and you can make it full-width. Click here to learn more.
If you’ve stretched your element to full width and still have a gap below it, try dragging the next section up right below the gallery as explained here . You may need to remove or drag any elements that will block you from dragging it up.
It’s possible to create a responsive site with code but it will require a bit of work. I was able to find another forum post for you that may help you out.
https://www.wix.com/corvid/forum/community-discussion/responsive-site-layout
Hope this was helpful!