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As far as the second element "ham" is concerned you may be right (I don't really know anything about Sanskrit so you could tell me anything). This is probably what you wanted to say. Th first element "Gra-" may be derived from a personal name GRANTA or else from OE "gran" (gravel). This is at least what the Oxford Dictionary of English Place-names says.
Neither the Oxford Concise Dictionary of English Etymology nor the German dtv Etymologisches Wörterbuch des Deutschen mentions the Sanskrit word though.