While the naming of the color gainsboro is uncertain (many of the
Unix colors either have very
bland names or rather peculiar names) one possibility is that it comes from the artist
Thomas Gainsborough.
Thomas Gainsborough lived from 1727 to 1788 in England and is most well known today for his landscapes and portrait paintings. He was regarded as a specialist landscape painter initially and there is some evidence that he worked in collaboration with other painters initially by adding to the background of portraits by other artists. With portraits, his work was very 'natural' and abandoned the 'conversation piece' style that was popular them, instead drawing direct head and shoulder portraits. He was toasted by one of his artistic rivals (in portrait painting) at the Royal Academy as "Gainsborough, the best Landscape painter in England" to which another replied "... and the best Portrait painter too."
His paintings are often a blend of people and landscapes. As a personal commentary, while the people are very lifelike and expressive, I am in awe the trees and sky that he paints.
Within rgb.txt, Gainsborough falls between gray86 and gray87 (the gray scale goes from gray0 (black) to gray100 (white)).
name hex r g b
gray86 0xDBDBDB 219 219 219
gainsboro 0xDCDCDC 220 220 220 (standard HTML Color)
gray87 0xDEDEDE 222 222 222
To see some examples of his works:
http://www.fiu.edu/~hastyd/gainsboro.html
I especially recommend glancing at his portrait of Johann Christian Bach and compare it to other late 1700 portraits to see a clear and clean break from the traditional style at the time.