I have already replaced the stock photo gallery app on my Sony Xperia Z1 with the new Google+ photos app. It is so good, and I live and breath Google+ so why not? You have the photo editing features directly in the app, you have all your photo folders and albums, you have a new share menu where you can share to other social networks and services.
I am sure that Google will replace the Stock Android gallery app, with Google+ Photos in the future. It’s quite obvious. In Android 4.4 KitKat Google Now (Search) actually is the launcher, the next step is to replace parts of Android with other Google services and apps. Google+ Photos could be the next step.
All photos uploads automatically to Google+, so you can easily see which photos that is currently uploaded by a small cloud icon on the photo thumbnails, but you can of course also choose photos from your phone that is not uploaded yet, and edit or share them too.
I like the design of the new Google+ photos app, and I can highly recommend you to use it as your default gallery app.