I think Gambler A might be too strong because it doesn't seem that hard to manipulate - just be happy with 2 cards if you succeed, and know what kind of card dominates your deck. If you pull a Victory card the first time, you can pretty much try for a second card for free - if you pull 2 Victories and thus have to discard them, that's usually as good as if you'd actually drawn them with a Lab - sometimes better (Library), sometimes worse (Crossroads). It'd be less than great in a BM deck since it'd usually pull Treasure (and you wouldn't want to risk trying to draw a Victory card second), but it'd be a little bit of insurance once you start greening, I guess. Engines - who knows, very swingy. But I like the idea of a card that gives you a chance. There'd be some AP, but only in situations where the player is starting to become desperate anyway.
Gambler B seems crazy. Triple-Lab if I get lucky and basically nothing otherwise? Or I could hit the "Jackpot" (I take it you're going for a slot machine theme?!) and draw my deck? That is a whole lot of gamble. (Also amazing for alt-VP, especially Silk Road.)
How about something like...
+1 Action
Reveal the top three cards of your deck. If they are all
- Treasure cards, +$3, +1 Buy.
- Victory cards, set them aside (return them to your deck at the end of the game); and +3 VP.
Put the revealed Action cards into your hand. Return any remaining cards to the top of your deck.