Gambling 3NT is a preemptive opening bid. The bid is used to describe a hand containing a minor suit of at least 7 cards in length and headed by the AKQ at minimum. The bid has the twin intentions of interrupting the opponents' bidding if they hold the majors and trying for a cheap game in notrump if partner provides adequate support.
Partnership agreements vary as to whether this bid shows a side ace or king. The original version of this convention required the opener to have at least two stoppers in the side suits. This is now known as a Strong Gambling 3NT. The most common current variant is that the bidder denies any stopper in a side suits (Weak Gambling). Other variants include having at least one ace in a side suit, or exactly two aces in the side suits.
Because this conventional bid replaces the standard use of opening 3NT (a hand with 25-27 high card points and a balanced distribution), partnerships must use other bids to show the standard 3NT hand. For partnerships which also employ the use of the strong two club convention, that bid can be used to describe a standard 3NT opening hand. For example, the following bidding sequence:
replaces the traditional 3NT opening bid if gambling 3NT and strong two club is in use. That is, North's hand is best described as 25 to 27 high card points and a balanced distribution. (See also Kokish for yet another way to show this hand.)
Responses vary depending on the agreement for stoppers in the side suits.
For the strong 3NT variant, the responder will normally pass unless holding a strong enough hand to envision a possible slam.
For the weak variant, responder will pass when holding a stop in three suits or with two aces (3NT becomes the contract), and otherwise will bid 4♣, 5♣, 6♣, or 7♣. This bid should be passed by the opener if the minor is clubs or corrected to diamonds. If partner responds 4♦, opener bids their shortness (singleton or void) if it is a major, 4NT with 7222 shape, and their long minor suit with a singleton in the other minor. With four sure tricks, responder can also bid 4NT, asking opener to continue to slam with an eight card suit.
A double by either opponent shows a strong hand which can expect to defeat 3NT with moderate help from partner, and invites partner to take out with some shape if the opponents run to their long suit.
If 3NT, doubled or otherwise, becomes the final contract, it is recommended that the opening lead be an ace, in order to see the dummy. A typical reason not to lead aces against other contracts is that it may give away a trick when opener holds the king; here that is not possible. The reason that the ace is led is that the offense may have nine top tricks, and the defense must take its five first, without losing the lead.