To add photos to an existing gallery on a WordPress page:

1. Click on the 'Pages' tab.
2. Click on your gallery page.
3. Make sure the 'Visual' tab is selected.
    
4. Click on the gallery in the page content area.
    
5. Click on the 'Edit Image' icon that appears on the gallery.
    
6. Click the 'From Computer' tab.
7.  Click 'Select Files' and upload the file(s) form your drive.
8. Click the 'Save all changes' button at the bottom of the window.
9. Close the gallery window.
    
10. Click the blue 'Publish' or 'Update' button to save your changes.