NASHVILLE — MAKE no mistake: This is still Guitar Town, and the cavernous Nashville Used Music store is proof. Here, amid rows of new and used six-strings, one finds country music veterans, hirsute rock dudes and honky-tonk strivers picking away most hours of the day in a gloriously dissonant jumble of twang. But in a back corner, co-owner Charlie Shrader has been stocking, in ever-growing numbers, the gaudy symbols of the new Nashville: the Gabbanellis. That is, Gabbanelli brand push-button accordions — bright, spangled things, some tricked out in the red, white and green stripes of the Mexican flag, and...