I want to open a file then move to the next one with a keystroke.
I don't want to install a separate image viewer only for this.
Your question is like asking how to use Photoshop as a simple image viewer. As Sampo said, GIMP is simply not designed for it. Using GIMP in this way is like using a car to go from the kitchen to the bathroom. You can do it, but it isn't really worth the trouble and it doesn't make the trip any easier.
Depending on what Linux distribution and Desktop Environment you use, you almost certainly already have an image viewer installed. Here are the defaults for some of the most popular Desktop Environments.
One of these is sure to already be installed in your system.