If you're looking to connect an existing gallery to another, this help article provides a step-by-step guide to walk you through the process.
Steps
Go to the Gallery you want to connect other galleries to.
Click the Menu button at the top right (the purple circle with 3 white lines)
Scroll down and Select "Linked Galleries" on the menu list
Click on "+ Link sub gallery" to connect a gallery
Enter the button title: This will help guests know what the linked gallery is to help them navigate. Try to keep it short.
Toggle on "Link existing gallery"
A dropdown will show, select the gallery you want to link
Click the "Set" button
Repeat the process until you've linked all relevant galleries, then "Save changes"