Steps to add an existing NextGEN Gallery to a WordPress page:
Go to Pages > Add Page.
Navigate to the Visual editor and click the green box (“Attach NextGEN Gallery to Post”) to open the gallery pop-up.
In the “Select a display type” section, select the way you would like your your gallery to look on the page. I am partial to the Horizontal Filmstrip (available only if you have the NextGEN Gallery Pro).
In the “What would you like to display?” section, select the gallery that you would like on your page.
Click Save to insert it on your page.
Click “Publish” – you now have a page with a NextGEN gallery!