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1. The Gap Concept states that you should call a preflop raise with a better hand than you would raise with in the raiser's position.
The Gap Concept doesn't state that.
The gap concept is that you should call with a tighter/stronger range than you'd use for an opening range in the same position.
e.g. You'd typically open AJ in middle position, but if someone opens the pot before you, you shouldn't call with AJ.
To put it another way, you might open 20% of hands in the hijack seat if the action is folded to you, but if someone opens before you, you might only play 10%.
The gap concept is specifically useful when shortstacked and your only good options are shove or fold. You might be in a spot where you'd open jam 25% of hands if you get the chance, but if someone else jams before you there are very few hands that can profitably call. This is the case because the opener can leverage fold equity (he makes money from getting folds), but a caller can only win by making the best hand. You'd usually need your calling range to be about half the size of the jammer's, since even the worst hand you call with has to beat 50% of villain's range.