How to repair garage interior back wall damage hit lightly by my car
It is almost certainly drywall (also called wallboard or plasterboard), which usually consists of a gypsum based core with paper faces on each side.

The damage looks only cosmetic. You could cut out the damaged section and replace it with a patch and then tape and mud it. However, it looks shallow and intact enough to just fill in over it.

You need drywall tape, preferably the self stick mesh type, and drywall compound (also called mud), a filling material.


Cut pieces of tape longer than the hole and cover it horizontally, overlapping pieces of tape so the hole is covered and the tape extends at least 1/2 to 1" on each side.
With a taping knife, apply the taping compound, gently pushing it through the tape. Perfect filling it not essential. Overlap the compound on to the sound area of the wall for at least an inch or two. Do not apply too much mud on the first coat.
Allow to dry. Wipe down the compound with a damp sponge (a form of wet sanding) to remove high spots and any lines.
Reapply another coat of compound, covering a slightly wider area. Again allow to dry.
Wet sand again. If even and smooth enough, allow to dry and repaint. If not, you may want a third coat or additional smoothing.


Note that any painting, even with the exact same paint originally used, will not fully blend with the existing coat due to fading/aging. If an invisible patch is desired (probably not necessary in a garage), you may need to repaint the whole wall or area.