Gnome Shell places pop up notifications on the middle bottom of screen. Some people prefer the Unity and Gnome 2 style notifications at top right corner

Here’s a gnome shell extension called Shell OSD which can place shell notifications in the upper right hand corner.

The gnome shell default notification:

After enable Shell OSD extension:

To install Shell OSD extension:

In gnome-shell using Firefox web browser to install and enable this extension at this page: