Most of the sleeping gasses have undesirable side effects. Considering the gang wanted the students unaffected, Anya was certain that they wouldn't overdose on them. So she has to hold her breath as long as possible and avoid breathing it directly.
And also, she can't appear level-headed when the driver was excessively attentive to them and Jackson was freaked out. So she flustered, "What is this white gas?" It also served as an alert to Dot.
Then began her battle of holding her breath. Meanwhile, she moved her hands and slammed the section separator and windshield.
Dot took a moment to discern the white gas before he desperately asked her, [Shall I take action?] He could stop the car, use the cops and get them out safely.
Being unconscious is dangerous in a life-threatening mission. Not to mention, she wasn't in complete disguise and Miles Johnson might identify her easily.
