G-Case Aims at Making the Nintendo Switch More Convenient for Gaming on the Go - IGN - IGN
Outside of adding additional protection to the back of the console, the G-Case includes a 5000mAh battery pack that provides up to 5 hours of additional battery life and the option to detach the battery pack and use it to charge other devices such as your smartphone or a tablet.
To minimize the number of items you are carrying, the G-Case also includes two cards slots: one on each side of the grip cases, allowing you to hold up to three Nintendo Switch games with the attachment.The G-Case also allows you to detach the case and Joy-Cons themselves from the Switch.