A Gauge is a value that is sampled at some point in time. Typical examples for Gauges would be the size of a queue, or the number of threads in a running state. Since Gauges are not updated inline when a state change occurs, there is no information about what might have occurred between samples.
Consider monitoring the behavior of a queue of tasks. If the data is being collected once a minute, then a Gauge for the size will show the size when it was sampled (a.k.a. last-write-wins). The size may have been much higher or lower at some point during interval, but that is not known.