$3 or more makes it so much better than Saboteur. Used early game, a player might find it hard to hang on to a simple Silver.
Trashing a card that is at least as expensive feels odd. How often will Peddler and Gambling Hall be in the same game? And if an opponent doesn't have such a card of equal cost, you'd basically be trashing a good card from your hand for nothing.
at the moment Gambling Hall is a bit like a Bishop/Treasure. GH obviously doesn't give you VT, but the spend is +$1 more and requires no Action to play. More than one of these would clean your deck faster than other players so the 'trash one card per round' benefit to others seems worth it. Maybe tweaking it slightly would be more balanced ($3 feels too cheap):
GAMBLING HALL, $4, Treasure-Attack
+$2
When you play this, each other player with at least 5 cards in hand trashes a card from his hand.
You may trash a card from your hand.