This extremely rare name is what is technically known as an 'Anglicization'. In effect this means that it is a phonetic representation of the originally sound, or as near as (in this case) the English pronunciation and spelling can achieve. In the early surname period (circa 1100 - 1300) it was common to 'Anglicise' French name i.e., Gervase to Jarvis, Paneterie to Panter etc., however with Gensavage we have a 20th Century example. The origin is probably either Polish or Lithuanian, the development being from 'Janowski or Januzewics' or similar, meaning 'the son of John', the latter being the most popular surname and christian name in Europe with some three hundred plus spellings form Evan, Ivan, Ian, Johan and Jen to the British Jones and Johnson. The only known 'original; example is shown below. The first recorded spelling of the family name is shown to be that of Frederick Gustavous Jankowski, which was dated November 18th 1852, married Eliza Haber at St. Pancras Old Church, London, during the reign of Queen Victoria, 'The Great White Queen', 1837 - 1901. Surnames became necessary when governments introduced personal taxation. In England this was known as Poll Tax. Throughout the centuries, surnames in every country have continued to "develop" often leading to astonishing variants of the original spelling.