One of my relations a brahmin girl married to a christian who has adopted hindu religeous practices. During his grihapravesam ceremony ,the vadhyar asked for his gothram and he was stumped. His wife covered up for him lending her gothra for the ceremony.LOLMy friends especially brahmin friends have nicknamed me as kristava brahmanan, since i speak tamil as they speak, i am a vegetarian, i wear dhothi as panchakacham, i read scriptures always, i mingle with brahmins, i learnt sanskrit and above all these, my wife is a brahmin.
To the extent i know, a brahmin is one who seeks brahman, the eternal truth (unmai paramporul), who has realized brahman, who practises brahman