A descendant being a grandson or a still lower offspring is called gotra,
When an offspring, with reference to a person, is the son's son of that person or lower than that, it is called Gotra. Thus the son of 'Garga' will be गार्गिः, and the son or grandsons &c of गार्गिः with reference to Garga will be गार्ग्यः ॥ Similarly वात्स्यः ॥
Why do we say 'a grandson or a still lower descendant'? The immediate descendants or the son will not be called Gotra. Thus कौञ्जिः, गार्गिः ॥,