Drywall as a partition wall in an unheated room?

Not bad that thing, but isn’t it a bit of overkill? I would simply use OSB.

Definitely overkill :) it depends on the requirements, I can understand why someone wouldn't like OSB. It certainly serves its purpose but visually it's not everyone's thing (even when painted )

Agree, I wouldn't want it in the partially inhabited garden house or in a Hot Yoga hut either, but in the external storage? Alternatively, cover it with vinyl wallpaper or foil...