Gallons contain halogens, which are highly reactive with oxygen

Halon is a general term for any of the several compounds that are composed of carbon, possibly hydrogen and halogens.

Generally, halogens are quite reactive while halons are found to be quite inert.

Halons that reach the upper atmosphere can break down under the ultraviolet light, releasing halogen radicals, for which 'highly reactive' will be too mild a term. However, they do not really react much with diatomic oxygen.