The Gap Concept is one of David Sklansky's poker theories that states a player needs to have a stronger hand than what they would raise with in order to call a raise. The concept highlights the fact that poker players actually prefer to avoid confrontations with another player who has indicated they have a strong hand and also that calling only has one way to win a hand (being the best hand) whereas raising allows you to win immediately if your opponent folds.
The majority of the players taking part in the GSOP $100000 Betfair Poker events will have some understanding of the gap concept but a much smaller sample size will be aware that the size of the gap is not fixed, in fact quite the opposite is true. Some of the factors that alter the size of the gap include where at the table the raise came from, the size of the chip stacks in play and the playing style of the player making the raise.
It should be obvious that the gap, or the difference in strength of cards needed to call the raise, widens when the player making the raise has a tight range of hands that he plays. This is because when a tight player enters a pot he is more likely to hold a premium hand. Conversely, a player with loose starting hand requirements actually narrows the gap as they have many more weaker hands in their raising range.
Again, if the raise comes from early position then you should tend to give it more credit for being a strong hand than say if the raise came from one of the later positions at the table. Likewise, if you are facing a raise from a deep-stacked player in one of the Betfair GSOP 6 events then the gap should be narrower as they is an increased chance they are simply splashing around in the pot but a standard sized raise, and not an all-in bet, from a short-stacked player should be treated as immense strength, which widens the gap.
So to recap, the tighter the player and the earlier position the raise comes from the wider the gap, therefore the stronger the hand required to call becomes, but a looser, late position raiser narrows the gap so you can call with a weaker hand. Simple when you think of it really!