If you have photos in your Google+ account that are not on your device or Picasa, you won't be able to see them in the Gallery, only in the Photos app.
Gallery only shows what's actually on your phone. The default view of Photos shows everything that is in your Google+ Photos account. If you use the left slide-in menu on Photos and select the "On Device" option you'll see the same stuff that Gallery shows. You can download individual photos to your phone if you need to work on them with an app that only works on your phone.