Original Wording

Mind-Numbing Gas: Opposing characters that occupy a square with a hindering terrain marker placed by Mysterio can't use Indomitable, Super Senses, or Willpower if they began the current turn in that square.

The Grand Reveal: Give Mysterio a power action and choose a friendly character with a single base. Mysterio and the chosen character are placed in each other’s squares. Neither character can then be given another action this turn.

Errata

Mind-Numbing Gas: Opposing characters that occupy a square with a hindering terrain marker placed by Mysterio at the beginning or end of an action can’t use Indomitable, Super Senses, or Willpower if they began the current turn in that square until the end of that turn.

So how exactly does Mind-Numbing Gas work?
In a nutshell, if an opposing character is in a Smoke Cloud square placed by Mysterio at the beginning or end of an action, the trait activates. This means that even if a character is given a move action to move out of the Smoke Cloud, they will still be under the influence of Mind-Numbing Gas since they began an action in the Smoke Cloud. Once this trait has been activated on a character, that character is under its influence until the end of current turn.

When a character has been affected by Mind-Numbing Gas, it can't use Indomitable, Super Senses, or Willpower. Depending on which player is the active player, this has some different possible ramifications. If Mysterio's controller is the currently active player, chances are the inability to use Super Senses will be the most critical, as Mysterio and his pals will probably try to attack the Mind-Numbed characters. On the flip side, if it is the opposing player's turn, the lack of Indomitable and Willpower will most likely be the biggest deal since the player will have to choose carefully about whether or not he wants to push his characters. Mysterio's controller might benefit from the lack of Indomitable and Willpower, too, since it could help make attacks using Incapacitate more potent.

If I counter Mysterio's Smoke Cloud power, will my character (that had been sitting in the Mind-Numbing Gas) regain the use of Indomitable, Super Senses, and Willpower?
Not this current turn, no. While countering the power Smoke Cloud would remove the hindering terrain markers from the map, the trait Mind-Numbing Gas does not require that the hindering terrain markers stick around for the entire turn. As long as an opposing character occupies a Smoke Cloud square before or after an action resolves, the Mind-Numbing Gas will continue to function no matter what happens to the Smoke Cloud tokens themselves. So you can counter Smoke Cloud to keep Mysterio from placing more Smoke Cloud tokens on his next turn, but any characters that are currently under the influence of the trait will remain so until the end of the current turn.