I have an exterior drywall, it is a kind of fiber cement panel, wall that has been bitten by a car bumper.
I know how to fix interior drywall, but have never worked with the exterior stuff before.
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It is exactly the same. I am guessing you are referencing the drywall that is installed in your garage. This is regular drywall - probably 3/4".
If it is a large dent you may need to cut out a rectangle to the studs and new piece. If it is small just some joint compound and smoothing.
A general tip for garages is that you prime them thoroughly, then use an oil based paint (yes boooo), and then add bumpers to the walls you may hit. These can be fancy - search "garage wall bumpers" or can just be some 1/2 x strip lumber.