Evry child knows the Gemara Ta'anit 21a, "ואמאי קרו ליה נחום איש גם זו דכל מילתא דהוה סלקא ליה אמר גם זו לטובה," "Because whenever something untoward happened to him he would say: "This, too, is for a good purpose."
Normally the construction "ish such-and-such" indicates place of residence, and the Aruch pointed out that גִּמְזוֹ, Gimzo, is a place listed in ii Chron. 28.12: "גמזו. בתענית בפרק סדר תעניות בגמרא נחום איש גם זו י"מ שם מקום ואמר בדברי הימים יששם אחת מן העיירי' גמזו שמה."
Some have suggested that the Gemara's explanation must have been absent from the Talmud used by R. Nathan, author of the Aruch, because it explains the name based on his trait of saying "gam zo le-tovah" and he doesn't even mention it. Furthermore, Sefer Yuhasin quotes the Aruch followed by R. Nissim Gaon, and he simply gives the Gemara's explanation. Therefore it has been suggested that R. Nissim's explanation later became incorporated into the text of the Gemara from marginal notes (likely he had some source or tradition, or perhaps he just based it on the story, which quotes him as always saying gam zo le-tovah. One might even say that the story is clearly alluding to his name, even if the Gemara didn't spell it out.).
Whatever the case, it occurred to me that maybe he really was called "Gam zo," (or "Gamzo") rather than "Gimzo," and perhaps the Gemara (so to speak) or R. Nissim, being well aware of ii Chron. 28:12 was really asking how come his name is pronounced gam rather than gim. Not "why is this his name?" but "Why is his name pronounced this way?"
(I started writing this post last night, but then I looked in the edition of the Tishbi with the commentary Raglei Mevaser printed in 1910 and I saw the author, R. Eliezer Herstick (sp?) literally says exactly what I was thinking, although he takes it as an authentic original part of the Gemara.)
But I further wonder if this isn't a phenomenon of vowel shifting between /a/ and /i/, c.f., aben versus ibn (see my post) or names like miriam and mariam (Μαριαμ, in the Septuagint). There are many more examples.
Graetz made a very interesting and possibly compelling suggestion that Nahum ish Gimzo is identical with נחמיה העמסוני, mentioned in Pesachim 22b and Kiddushin 57a. In both places the Gemara first calls him Shimon, and then Nehemia. In other words, it is uncertain even of his name. Graetz points out that we know nothing of this person, but his teaching method is similar to that adopted by Rabbi Akiva, otherwise known as Nahum's student (or in one version, the teacher-student relationship was reversed). Furthermore, Hagigah 12a attributes the same exegetical method (explaining each particle et) to Nahum. Also a very guttural ayin is similar to gimmel, close to or identical with the letter ghayn in Arabic, and there are several examples in the Septuagint of words spelled in Hebrew with an ayin being transcribed by a gamma. The upshot is that עמסוני may be identical with גמזו (or, if you like, גמזוני). Graetz further explains both as the same as the place known by the Greek name Emmaus. See the Monatsschrift für Geschichte und Wissenschaft des Judentums, Volume 19 pg. 527, and others accept this identification as well.