Google Photos Add a Location on Multiple Files
This can only be done through desktop on Picasa web albums as far as I know, but Picasa shares photos with Google Photos, so it should work.


Put all your photos with the same location in an album (you can do this with Google Photos)
Go to Picasa Web Albums and find the album.
Under "Actions", click "Album map"





This will bring you to the album map. Once there, click "Edit map"





Click the first picture in the album to select it, then while holding shift click the last picture in the album.





Drag the pictures out onto the map. You may want to find your location first.