Definition of gaper
When we talk about "gaper", we are talking about the hole in one hand, the hole in sequence. The expression is used to talk about private cards that we receive preflop. If we receive 7-8, we talk about connectors. If we receive 6-8 or 7-9, we talk about "gaper" in this case, 1 gaper (a hole of 1 to the sequence). We can also describe a hand as a 6-9 "two gaper". Gapers are generally less interesting to play than connectors because they will hit less flop, but they should not be undervalued.
Some gapers dominate connectors
A hand like 9-10 in hearts is a matching connector. We know that this type of hand will play very well. Against a random hand, this hand will have 54% equity. If we now take a one-gaper (J-9 in hearts), we have a matching gaper rather than a connector, but we now have 55.6% equity against a random hand. This is usually referred to as a "one gaper" or "two gaper". The term "three gaper" could be used, but it almost never is (three gapers are not as strong as one gapers or connectors). Gapers are also hands that will make less obvious straights to your opponents.