Sometime between 1032 and 1064, a miller named Gualdrich, his wife Richilde, and their three daughters, Adelaide, Dominica, and Alburg, were indentured to the abbot of Marmoutier Abbey in Tours, and a document was drawn up in witness of this act.  An ordinary sort of activity — Marmoutier had many serfs, especially from the 11th century onwards when Marmoutier was one of the richest abbeys in Europe — and an ordinary sort of document — with many serfs come many records.
So what’s so interesting about this particular one? It isn’t anything to do with Gualdrich or Richilde or their excellently named daughters, but rather someone else who is mentioned in the document, namely, one of the witnesses, one Guanilo monachus.
“Guanilo”, you, well-versed in medieval philosophy, say. “Huh. I recognise that name. But, surely…”
Surely it can’t be that Gaunilo. Surely it can’t be the one who refuted Anselm’s ontological argument. I mean, he is from the 11th C, but we don’t really know anything about him, do we?
Well, that’s what I’d always assumed; I’d never actually read up on Gaunilo myself. So I did what any self-respecting academic would do, and plugged “Gaunilo” into wikipedia. Turns out, the conventional wisdom is that the Gaunilo of best-island-ever fame was a Benedictine monk of Marmoutier Abbey, in Tours, France. I.e., the guy who also witnessed the indenture papers of Gualdrich, Richilde, and family.
Little is known about the author of the rejoinder to Anselm. The Liber pro insipiente objection to Anselm, written around 1079 , is the only philosophical writing that has been attributed to him. (This rejoinder to Anselm is available in English translation from the Medieval Sourcebook , but I’ve not yet found a Latin edition easily accessible. ) In 2009, Ian Logan questioned whether the author of the Liber pro insipiente was in fact Gaunilo of Marmoutiers:
Neither Anselm nor his biographer, Eadmer, refer to the author of the Pro insipiente by name. Anselm appears to be unaware of the identity of the author, given the manner in which he ddresses him at the beginning of his Responsio: “Indeed you say — whoever you are…’. Eadmer refers to Anselm’s opponent simply as ‘someone’. 
Logan traces the basis for the ascription of the work to Gaunilo to a single early manuscript from the late 11th/early 12th century from Jumièges, now at Rouen (MS 539 (A366)), where “Gaunilo monachus Majoris-Monasterii” is given as the author (it is the only early MS to do so). Logan says that “as there is no separate manuscript tradition for the Pro Insipiente to that originating with Anselm at Bec (not even at Marmoutiers), it is possible that this ascription is the result of a scribal fantasy” .
But why would some scribe pluck an attribution like this out of thin air? Gaunilo is not a common name in 11th C France (or indeed later), so an explanation whereby the scribe picked a ‘generic’ name for the author of the text cannot be admitted.  Why, too, would the scribe specifically locate the author at Marmoutier?
Both of these seem to be unlikely choices for someone to simply make up. We aren’t, here, however, in the position of being able to address the question of whether Gaunilo, the monk at Marmoutier, was the author of the Pro insipiente or not. Let us rest content without further questioning the attribution of the work to this Marmoutier Gaunilo, and instead ask: What else can we find out about this Gaunilo from the indenture records collected in ? For the charter with which we opened our discussion is not the only one that “Gaunilo monachus” is mentioned.
He next appears in charter LXVI, dated to 1062, witnessing an indenture record concerning Fulcher of Tours and his sister Agnes. The next year he is mentioned in another document of a much more personal nature, for it concerns the nephew (and namesake), Gaunilo, of Gaunilo the monk (domni Guanilonis monachi nostri), in charter CIII.
In all three charters, the editor of  is confident that the “Guanilo monachus” mentioned is the same one — this is not an unreasonable assumption given that Gaunilo or Guanilo was not a common 11th C French name, and therefore the likelihood of there being more than one monk by that name at Marmoutiers is small. (There are other people named Gaunilo in these documents; but those that are identified in secular contexts cannot be equated with the monk.) The fact that the monk Gaunilo had a nephew named after him allows us to find him in a few other charters as well (that is, ones where he’s not explicitly identified by the editor). In charter I of the appendix, again dated between 1032 and 1064, there is a reference to one Guanilo Thesaurarius (i.e., treasurer). Later in the same charter is a mention of Guanilo nepos Guanilonis thesaurarii ‘Guanilo nephew of Guanilo treasurer’. The fact that we already know (from charter CIII) that our Gaunilo had a nephew named Gaunilo makes it almost certain that these two are the same, so we can conclude that not only was Gaunilo a monk at Marmoutier, he had, at some point, held the office of treasurer. Gaunilo the treasurer-monk turns up again in appendix charter VIII, dated 1040-1044, as both a witness and as the “owner” of the charter: after a lengthy opening, we get the phrase Proinde ego Guanilo, thesaurarius Sancti Martini ‘Hence I, Gaunilo, treasurer of Saint Martin’. But here, he’s not the only Guanilo:
Nihardus de Monte Aureo nepos meus, sed et alius nepos meus, Guanilo nomine, filius Gauscelini, nec non et alii duo nepotes mei Cleopas et Guanilo filii Malranni de Castor Noiastro, cum sorore ipsum, nomine Hersindi, uxore Adelardi Barduni.
That is to say:
Nihard of Mons Aureus my nephew, and another nephew of mine, by name Gaunilo, son of Gauscelin, and yet further two other nephews of mine Cleopas and Gaunilo sons of Malram of Noyers, with their sister, by name Hersinde, wife of Adelard Bardun.
This means that in appendix charter XX, dated around 1063, when we find one Guanilo de Montiniaco (i.e., of Montigny) and a reference to Guanilo thesaurarius avunculus ejus ‘Guanilo treasurer his uncle’, this is either Guanilo son of Gauscelin or Guanilo son of Malrann.
So not only do we have Gaunilo the treasurer, who is almost certainly the same as Gaunilo the treasurer-monk, who is definitely the same as Gaunilo the monk, we have a whole bunch of his relatives too. How cool is that?
 André Salmon, ed., Le Livres des Serfs de Marmoutier, Publications de la Société Archéologique de Touraine XVI (Tours: Imprimerie Ladevèze, 1864), charter LV.
 Jasper Hopkins, “Anselm’s Debate with Gaunilo”, ch. 4 of Anselm of Canterbury, vol. 4: Hermeneutical and Textual Problems in the Complete Treatises of St. Anselm (Toronto and New York: Edwin Mellen Press, 1976), p. 97.
 It’s a modernized version of the translation published in St. Anselm: Proslogium; Monologium: An Appendix In Behalf Of The Fool By Gaunilo; And Cur Deus Homo, Translated From The Latin By Sidney Norton Deane, B. A. With An Introduction, Bibliography, And Reprints Of The Opinions Of Leading Philosophers And Writers On The Ontological Argument, (Chicago, The Open Court Publishing Company, 1903, reprinted 1926). Another translation, due to Jasper Hopkins, is available here, along with Anselm’s rejoinder.
 The Latin text appears in S. Anselmi Cantuariensis Archiepiscopi Opera Omnia, edited by Franciscus Salesius Schmitt, O.S.B. (Stuttgart-Bad Cannstatt: Friedrich Frommann Verlag, I968), Tome I, Volume i, p. I28, as well as in Jasper Hopkins, A New, Interpretive Translation of St. Anselm’s Monologion and Proslogion, (Minneapolis: The Arthur J. Benning Press, 1986) p. 264–275, and  below.
 Ian Logan, Reading Anselm’s Proslogion: The History of Anselm’s Argument and its Significance Today (Farnham: Ashgate, 2009), pp. 115-16. Thanks to Mark Thakkar for the reference.
 A much more generic choice of name for 11th C France would have been, e.g., Guillaume.