Jayarava's Raves: Some Additional Notes
2. The Name Gautama
In my essay What Was the Buddha's Name? I drew attention to the quirk of history which left the Buddha, a kṣatriya by tradition but possibly a non-āryan, with an ostentatiously Brahmin gotra-, or clan-name: Gautama (meaning 'descended from Gotama, the one with the most cows go'). However more than half a century ago D.D. Kosambi offered a different take on this subject in a review published in 1953:
D.D. Kosambi. 'Brahmin Clans'. Journal of the American Oriental Society, Vol. 73, No. 4 (Oct. - Dec., 1953), pp. 202-208.
He points to two brief Pāli passages which suggest that Gautama (Pāli: Gotama) is not the Buddha's gotra name. The first is from the Therīgāthā verses of the Buddha's maternal aunt and foster mother. She says (Th 2 162)
Bahūnaṃ vata atthāya, māyā janayi gotamaṃ;
Truly for the many, Māyā gave birth to Gotama
Kosambi's point here is that the names Māyā and Gotama are on the same level - i.e. they are both first names. This is to read the text quite literally, and I'm a bit doubtful about doing that. Compare for instance the case of the Brahmin boy Uppatissa, son of Rūpasārī, better known as Sāriputta 'son of (Rūpa)sārī'. However Kosambi points out that neither does the Buddha's wife become known as Gotamī in any tradition. The fact that Mahāpajāpati, his mother's sister, is called Gotamī also suggests that it is not the Buddha's clan-name since the names pass pass down patrilineally (though I think Kosambi here is thinking in terms of Brahminical social rules which required Brahmins to marry outside their gotra). Kosambi also notes that bhikkhus are sakiyaputta not gotamaputta. He does not attempt to explain why the future Buddha might be named after Vedic sages however, which still strikes me as odd.
Kosambi's other text is the Pabbajjā Sutta [Sn 3.1] in which King Bimbisāra asks the Buddha where he is from. The Buddha replies that he comes from the country of Kosala, and:
Ādiccā nāma gottena, sākiyā nāma jātiyā;
Tamhā kulā pabbajitomhi, na kāme abhipatthayaṃ.
Called Ādiccā by clan, called Sākiya by caste [jāti]
I went forth from that family, not longing for pleasures.
The phrase only occurs once in the canon, but elsewhere the Buddha says that the Sākiya consider rājā okkāka their ancestor [Ambaṭṭha Sutta, DN 3, PTS D i.92-3] and Pāli okkāka is Sanskrit ikṣvāku a king of the ādityā [P. ādiccā] gotra. The suggestion then is that the Buddha's name was in Sanskrit Gautama Ādityā; and Pāli Gotama Ādiccā. The Buddha is also sometimes called Āṅgirasa which according to the Dictionary of Pāli Names was a tribe which included the Gautama gotra. My reading of some of the DOPN references suggests that āṅgirasa was being used as an adjective (e.g. 'shiny like the sun') rather than a name. Against the passage above Kosambi also cites the Mahāpadāna Sutta [Dn 14, PTS ii.3]
Ahaṃ, bhikkhave, etarahi arahaṃ sammāsambuddho gotamo gottena ahosiṃ.
I bhikkhus, now worthy, fully awakened, was of the Gotama gotra. 
This phrase occurs 3 times in the suttas, all in the Mahapadāna. Kosambi refers to this as "the first interpretation of Gotama as the Buddha's gotra name... obviously a late formation under Brahmin influence". Indeed it is so obvious that Kosambi provides no evidence for his conjecture, nor does he consider the possibility that both statements about gotra are "late formations". Contrarily we find the name Gotama being used in the last two chapters of the Sutta-nipāta which are generally considered to be the oldest layers of the Pāli Canon.
It is still a puzzle as to why the Buddha even has a gotra name, let alone a Brahmin one (which both Gautama and Ādityā are). He was not a Brahmin. I don't think Kosambi solved the mystery, but he provided an interesting additional view point. One last observation of my own is that though the Buddha meets Brahmins from many other gotra lineages, he never seems to meet a Gautama Brahmin. This is despite the fact that the two ancestors Gotama and Bharadvāja are mentioned together in Bṛhadāranyka Upaniṣad 2.2.4, and Gautama the Buddha meets more than a dozen Brahmins from the Bhāradvāja lineage, who mostly seem to live in Kosala (see e.g. DN 3, 13, 27, 32, but throughout the nikāyas).