An elf is not immune to the Dragon's Sleep Gas
Jeremy Crawford has clarified in a tweet that Fey Ancestry only protects against magical sleep.
Nonmagical sleep gas/poison/etc. can knock an elf unconscious, unless its description says otherwise.
He also clarified in another tweet that a Dragon's breath is not magical.
The breath weapon of a typical dragon is not magical.
Unless it is explicitly stated in the Tomb of Annihilation text that this particular Dragon's breath is magical, the effect of the sleep gas would not be prevented by Fey Ancestry.
As mentioned by V2Blast, the Sage Advice Compendium answers the question "Is the breath of a dragon magical?" as below (emphasis mine):
Determining whether a game feature is magical is straightforward. Ask yourself these questions about the feature:
- Is it a magic item?
- Is it a spell? Or does it let you create the effects of a spell that’s mentioned in its description?
- Is it a spell attack?
- Is it fueled by the use of spell slots?
- Does its description say it’s magical?
If your answer to any of those questions is yes, the feature
is magical.
Let’s look at a white dragon’s Cold Breath and ask ourselves those questions. First, Cold Breath isn’t a magic item. Second, its description mentions no spell. Third, it’s not a spell attack. Fourth, the word “magical” appears nowhere in its description. Our conclusion: Cold Breath is not considered a magical game effect, even though we know that dragons are amazing, supernatural beings.
