This is a data area that can be found either side of a Block (aka Sector) or Track, though it does not indicate the separation like a Sync (aka Mark). The size of a gap area may be altered via mastering if allowed by the format. Any errors found after the gap can normally be ignored, though a few protections have been found that store data in this area.
Gaps are normally used to allow re-writing of specific blocks on a track. As this operation is not timing accurate, the gap provides the threshold needed to cover for the inaccuracies that occur in the timing of the write.
There is at least one gap between the last block on one track, and the first block of the next. It’s purpose is to compensate for the different properties of a track written on specific locations on the disk as well differences between drive speed alignment. If the drive speed is different within reason, this gap will have some of its content overlapping i.e., overwritten when mastering a whole track.