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Creating Gallery Page=>
Please create a simple page, Give it name “Gallery” and Click on right side setting icon, Select “Document” tab and find “Page Attributes” and select “Gallery 3 Col” template. Then click save this page.
Add This page in Site Menus=>
Add this gallery page in your website menus to to Appearance >> Menus >> Select gallery page and click on “Add to Menu” button and save this menu from right side.
Add contents for Gallery Page=>
After that, Go to Admin Dashboard Menus >> Portfolio / Project >> Add New Item >> Add Project title and description and create right side a category and check this for this gallery item, Also add featured image for this gallery post. Click on publish button.
Let me know your issue is resolved or not.