Abilities » Detention Gas
If Weezing has 10 HP left and is Poisoned, would it be Knocked Out before its "Detention Gas" Ability takes place?
No. Even if a Pokemon receives more damage counters from Special Conditions, Abilities, etc. than its remaining HP, that Pokemon isn't Knocked Out until "between turns" is over.
Source: TPCi Rules Team (2019-06-06)
Abilities » Detention Gas
My Active Pokemon got Knocked Out between turns because it was poisoned and Weezing now becomes the Active Pokemon; does its "Detention Gas" Ability work?
No, because you wouldn't replace your KO'd Pokemon until the between turn phase is over. When Weezing comes up, it's too late for Detention Gas at that time.
Source: TPCi Rules Team (2019-06-06)