Most Kenyan ethnic groups share a language. But Luhya, Kalenjin and Mijikenda are an exception. I think that is where the confusion come from. There is more to ethnicity than language. Maragolis and Bukusu are the same folks genetically, same to Marakwet and Nandi. But more importantly IMO is what is their consciousness of ethnicity? Do they think of themselves as one group? In that sense I think of GEMA as a potential "tribe" with Kikuyu, Meru, Embu, maybe even Akamba as sub-tribes - they just have not been made conscious of it.