Posted 2011-November-28, 01:55
First of all, I need to apologize for something. For most of this discussion, I've been thinking that we're talking about the opening side, not the overcalling side. So now that I've noticed my mistake, I need to look at the whole thing again.
South shouldn't be able to have 19 HCP. With that strong a hand, he should have doubled, not made a normal overcall. So the hand that overcalls and then jumps to 4♠ can't really exist. It's only in the bidding database to make sense of a human doing this, not for GIB to select a bid.
If South Passes on the second round, GIB's description is: No suitable call -- 5+ S; 8-17 HCP; 9-19 total points. So there's no gap, it's the same description as the overcall. The only correction we really should make is that it should be exactly 5 S, since with 6 it would bid 3♠ (it's just falling through to a default rule).
The LAW says that with the points about evenly split, and no compensating features, you should bid up to the level of your fit. In this case, NS only have an 8 card fit, so they should only compete to 2♠. EW have a 9-card fit, so they should compete to 3♥ (although the ♠Q and 4333 shape in West are flaws that reduce the total trick count).
There is a gap, though. If GIB has a 6-card suit and 13-18 TP, there's no bid in the book for this, because the LAW bid is limited to 9-12 TP, so the book says to pass. However, it turns out that simulations rescue us here. I moved a spot card from diamonds to spades in South's hand, and it usually bids 4♠ over 3♥. And if I make the spade suit a little weaker, but still 6 cards (AJT632), it bids 3♠. We should probably add rules to fill in the gap, so that the explanations will be more sane.