Gaps have no data in them. So the range 1-2 is a gap.
Clusters are groups of data that are right next to each other.
So the range 3-6 on your dot plot is a cluster, because there is a group of data there with no gaps.
Peaks are the greatest amount of data in the dot plot. 0 and 5 are the peaks because they both have the most data, 4.