The necklace of adaptation has the following effect:
While wearing this necklace, you can breathe normally in any Environment, and you have advantage on Saving Throws made against harmful gases and vapors (such as Cloudkill and Stinking Cloud Effects, inhaled Poisons, and the Breath Weapons of some dragons).
Now, harmful gases and vapors is very vague.
The question of what "harmful" means, was discussed in this question:
What defines a harmful effect or ability?
The first effect of the necklace was the subject of this question:
Does the Necklace of Adaptation let someone breathe under the surface of liquids other than water?
I could not, however, find information, on what counts as gases or vapors. The spells cited, cloudkill and stinking cloud are clear, as are "inhalded poisons" since every poison has an application property defining if it is inhaled. The "such as" and "breath weapons of some dragons" do show, however that it this extends to more effects, which are, at least here, not clearly defined.
Being a chemist, I’d rule that poison breath and acid breath, as well as dragon turtle breath would count.
Otherwise, if it says "gas" or "vapor" it probably counts, e.g. the faerie dragon’s euphoria breath.
I’m wondering, however, if there is anything better (preferably official) than "make it up".