Goop Gas IS as good as a lot of people think. As soon as it is played, all Pokémon Powers in play are nullified. It's like playing a temporary Muk, which disappears at the end of the opponent's next turn.

It's that phrase though, "the end of the opponent's next turn" that has obviously confused some people. I'll try to explain it, using two power-heavy decks facing off. Assume all pokemon have appropriate energy.

This "scenario" takes place mid-game. It's Deck One's turn, an Aero-Mew Deck. Aerodactyl, Mysterious Fossil, and promo Mewtwo on the bench, Mew with 10 damage active. Deck Two, a Damage Swap, has Chansey active, with Kadabra, Chansey, and tentacool on the bench.

Aero-Mew's Turn: After usual turn stuff, Mew uses De-evolution beam on Kadabra.
Damage-Swap's Turn: After usual turn stuff, uses Goop Gas attack, Aerodactyl's power is nullified. Uses Breeder on Abra, into Alakazam. (this point is nebulous; Either Goop cancelled all CURRENT in-play powers and has no effect on new ones, or all additional powers put into play within the turn limit are also cancelled. We're assuming the second option for now and will not move any damage counters around) Chansey Double-Edges Mew, KO.
Aero Mew's Turn: (Goop Gas still in effect) Promo Mewtwo placed in active. Evolves fossil into a second Aerodactyl (allowed to do so for same reason Alakazam could evolve; first aerodactyl's power is off). Mewtwo attacks, 10 to Chansey.

After Mewtwo attacks and before Damage-Swap draws a card, Goop Gas is no longer in effect, because after the opponent's attack, their turn ends. Goop Gas lasts from when it is played to after the opponent attacks (or indicates their turn is over)

So Goop Gas is good if you want to KO that Mime right away, or keep that Blastoise from powering up next turn. Its good if you want to keep the opponent from swapping their damage away, or you want to evolve. It is considerably more effective as a power-breaker than Dark Arbok, whose attack is more like "paralyzing" a benched Pokémon to keep its power from working for just the opponent's turn.