I don't have a lot of specific advice. I seldom use nx2 and never use Windows Photo Gallery. But I just opened photo gallery for the first time and I found all my pictures from the navigator pane.
In general, pictures are not located IN a particular photo organizer or editor. They are on your hard drive and the photo software merely displays them to you in an organized manner. Photo programs usually automatically find them and display them on their own although you might just need to navigate within the photo program to the proper location. On some programs, you need to do an import (like Lightroom). On other programs, you may need to tell the software what folders to monitor.
NX2 and Photo Gallery are of the 'navigator' style, if there are any photos on your hard drive, either program should display them when you navigate to that location. It helps if whatever you use to get the pictures off the camera puts them in a dated sub folder under My Pictures. Then they are easier to find.
If you process raw files, I'm not sure if one program will recognize edits applied by another program. If that is the case, you need to export the raw file as a standard type. A tiff file would be good if you plan further edits.
There might be an online gallery component to Windows Photo Gallery. I don't know if there is. It did ask me to sign in to something and I canceled. If you did have photos stored only online, they would not be available to NX2. Download them to your computer to a known location to find them with NX2.