These can be downloaded from various locations on the web. However, the downloaded files are almost always larger. GDI is intended purely for emulation, and is arguably more reliable. Once installed, click Back repeatedly, then Exit, to return to the main RetroPie menu. When you see the main RetroPie menu, use your controller or keyboard to scroll down to RetroPie Setup. While several emulators are included by default in RetroPie, some need to be added manually. If you have a Bluetooth controller, however, this can also be added with the help of the Bluetooth submenu on the main screen.
Detailed controller help with RetroPie can be found online. The Xbox and PS3 controllers are good options.
