Abba, the brother of Rabban Gamliel, [married the daughter of Rabban Gamliel, his brother, and he died, and Rabban Gamliel performed levirate marriage with his brother's co-wife (Yevamot 15a). It appears that he was the brother of R. Gamliel of Yavneh (see Mishnah Shekalim), therefore it is appropriate to mention him earlier in the order of generations, before Abba bar Abba the priest mentioned below]:
Seder_HaDorot,_Tanaim_and_Amoraim.1:1