In
baseball parlance, this is the space between the centerfielder and the two other outfielders. That is, the uncovered area of the field between the three outfielders.
Every baseball team, when it is in the field, therefore has two large "gaps" in the outfield. A batted ball that falls in this area and rolls to the fence is said to be "in the gap." Usually results in a baserunner reaching 2nd or 3rd base.
The gaps in the infield are referred to as "holes."
Major League baseball, in it's infinite quest to ruin the game and commercialize everything, has sold a great deal of advertising to the Gap corporation. In many stadia there are large blue signs, saying simply GAP, placed on the outfield walls in the area generally thought of as the outfield gaps.
I guess this is supposed to be cute or ironic. Try insipid or really dumb. Like the DH and interleague play it is baseball trying to be cute, and failing.