What do you call this plastic paneling on the outside of garage?
It is vinyl siding, available at big box stores. It will come in different sizes, double 4 or double 5, counting the inches between each horizontal bump. Some comes in double 4 1/2". Double means, there are 2 horizontal lines for each piece of siding.  There are other sizes available too, along with styles of the face that is created in the bending process.