Yes but spot-treat the rusted area only, as the Deox-GEL will remove the galvanization.
Deox-Gel will not harm sound automotive paintwork.
It is possible to spray Deox-Gel, but it is not recommended as the product must be applied thickly to the surface for it to work. deox-gel should not be thinned as it will compromise the function of the product.
Yes, Deox-Gel will work on vertical surfaces.
If possible, it is best to remove as much rust as you are able mechanically before using Deox-Gel.