Version-Release number of selected component (if applicable): gnome-software 3.10.4 / Fedora 20 final release. Reproducible: Just after the system installation. Steps to Reproduce: Search for the repositories configuration. Actual results: You can't find it. You need to install gnome-packagekit-installer or another software to be able to manage the repositories. Expected results: The system should offer the configuration GUI out-of-the-box.
This really isn't in the scope of what gnome-software is designed to do. If you want to enable repos such as updates-testing and things like that, there are command line tools to make that happen (yum-config-manager), as well as gnome-packagekit-installer as you noted.
In F21 it will again be possible to remove software repositories you have previously added; it's regrettable that this feature was not present in F20. It has always been possible to add new repositories by double-clicking an RPM; any other method (e.g. copypasting a URL) is too technical to expose in a user-friendly GUI.