Hi StephanS,
For a gallery you need to create 2 fields (Upload Image and Field X) Field x needs to call the field name given to the Upload image.
This is how I create my galleries:
- Create a images folder under joomla
root (images/seblod_gallery)
 - In Seblod construction Apps
Folder – create a folder called “Gallery”  to store gallery fields
 - Seblod construction click Fields
then filter to the new gallery folder you just created
 - Click New to add a field – choose
type “Upload Image” field name=Gallery
 - Under Folder * box type images/seblod_folder - the folder where images will be stored
 - Input Width
/ Height dimensions for thumbnails (example 150x150)
 - Save field –
standard – article – gallery
 - Create a new
field – Type – Field X
 - Name it - Gallery
Field x
 - Under Field * box type – gallery – or the name of the previous field
 - Under maximum,  type the maximum number of images that can be
uploaded to the gallery
 - Save storage as –
custom – article – introtext
 
Your gallery
fields are now ready
Insert both
fields in forms & content types (Gallery & Gallery Field X)
Hide the gallery
upload image Field – Label variation Hidden
When you create
an article the field x gives you the option to upload individual images for the
gallery
To create a gap
between the images on the front end add css 
.gallery-gap img
{margin: 5px;}
To make gallery responsive
create a js file and call it in the template
Or add this code in the
template
<script
type="text/javascript">
  // Make ColorBox responsive
  jQuery.colorbox.settings.maxWidth  = '95%';
  jQuery.colorbox.settings.maxHeight = '95%';
  // ColorBox resize function
  var resizeTimer;
  function resizeColorBox()
  {
  if (resizeTimer)
clearTimeout(resizeTimer);
  resizeTimer = setTimeout(function() {
  if
(jQuery('#cboxOverlay').is(':visible')) {
  jQuery.colorbox.load(true);
  }
  }, 300);
  }
  // Resize ColorBox when resizing window or changing
mobile device orientation
  jQuery(window).resize(resizeColorBox);
  window.addEventListener("orientationchange",
resizeColorBox, false);
</script>
Hope this helps
Anjo