Add Location to Photos
I'm afraid it's not possible using the standard google photos app, you would need to use some photo-processing software to set locations for photos after they were taken (Picasa comes to mind). Alternatively, you can use the G+ page https://plus.google.com/photos/yourphotos to edit the GPS information, but then again i'm afraid you can only do this one by one. Maybe it's best to just turn on the location feature while taking the photos and you're done