The problem actually is, I cannot see any additional custom fields while setting up Gallery. Screenshot is attached below.
http://prntscr.com/895eal
I have already checked all steps in FAQ section. Theme which im using is Spalab by Themeforest.
Demo: http://wedesignthemes.com/themes/dt-spalab/
The custom fields are visible and works for me. I have the latest version and use latest twenty15 and twenty15 childtheme.
My problem is that when I’m on the page and click on a thumbnail it won’t open.
I just want the thumbnails to open the full size on the same page as the gallery of thumbnails, hopefully I can find another solution for this.