An open-ended straight draw (OESD) can complete at either end and has 8 outs; a gutshot needs one specific rank in the middle and has only 4. The difference roughly halves your equity, and it decides which straight draws can call a bet.
With 6♥4♥ on K♦7♠2♣ 5♥, a 3 or an 8 makes a straight — an OESD with 8 outs, about 17% on the next card. With 7♥6♥ on A♠9♦5♣, only an 8 completes the straight — a gutshot with 4 outs, about 8% a card. A double gutshot, such as J♥T♥ on A♠Q♦8♣3♥, also has 8 outs.
The practical rules in this trainer follow from those counts. A bare gutshot facing a pot-sized bet folds: 4 outs are about 8% a card, and implied odds rarely cover a 33% price. An OESD facing a small flop bet continues, and an OESD plus a flush draw is strong enough to raise. The board changes borderline cases: on K♥Q♦6♣ the same J♠T♠ has 8 outs, every one of them to the nuts, and with a second card to come it is strong enough to continue. A draw to the low end of a straight can complete into a second-best hand, so treat it with the same care as a low flush draw, and remember that a gutshot on a board that hits the raiser’s range has no fold equity to add.
Worked example
FAQ
How many outs does a gutshot have?
Four — the four cards of the one rank that fills the gap. That is about 8% on the next card and about 17% by the river, which is why a bare gutshot rarely has the price to call a big bet.
Is a double gutshot as good as an OESD?
In raw outs, yes: both have 8, about 17% on the next card. Both still need the right price; the course folds J♥T♥ on A♠Q♦8♣3♥ against an 18-into-20 turn bet because 8 outs fall well short of a 32% price.