The first chapter of maseches Avos goes through the mesorah from Moshe Rabbeinu through the zugos who are all mentioned by name with no gaps, culminating with Rabban Shimon Ben Gamliel and his grandson Shimon ben Gamliel.*
My question is, why not list the leader ( let's assume it was Gamliel of Yavneh) in between the two? I can't assume he had nothing to say. There was a fifty year span of time from the death of Shimon ben Gamliel 1 until the birth of Shimon ben Gamliel 2. This was the period of time referred to as the second and third generation of Tanaaim, why just leave it blank?
The next chapter begins with Shimon ben Gamliel's son Rabi Yehuda HaNassi so again, there is no gap there.
*This last Shimon ben Gamliel is not the same personality as the one in the second to last mishna as pointed out by Tosafos Yom Tov. The fact that it writes his name and doesn't say הוא היה אומר should prove this correct.