I have a Blue Boy print signed in pencil by Gainsborough... right hand corner below print and it reads Blueboy at the left hand corner and at the top it reads Published by Edward Gross Co. Ny and the right hand corner reads Painted by Thomas Gainsborough. How can I find out if that signature at the bottom is authentic and if the print is worth anything or worthless?

Only one problem. During the time Gainsborough was alive they did not make prints of oil paintings and the signature in pencil could not have happened.

Thomas Gainsborough - Wikipedia, the free encyclopedia

He lived from 1727-1788.

They hadn't invented prints yet.

Sorry, it's worth the paper it's printed on and nothng more.

The Blue Boy is a very common print.

Gainsborough was a portrait and landscaper.

According to the history of printing - the following print method for oil paintings was used as early as the 1600's:

Mezzotint
Main article: Mezzotint

An intaglio variant of engraving in which the image is formed from subtle gradations of light and shade. Mezzotint—from the Italian mezzo ("half") and tinta ("tone")—is a "dark manner" form of printmaking, which requires artists to work from dark to light. To create a mezzotint, the surface of a copper printing plate is roughened evenly all over with the aid of a tool known as a rocker; the image is then formed by smoothing the surface with a tool known as a burnisher. When inked, the roughened areas of the plate will hold more ink and print more darkly, while smoother areas of the plate hold less or no ink, and will print more lightly or not at all. It is, however, possible to create the image by only roughening the plate selectively, so working from light to dark.

Mezzotint is known for the luxurious quality of its tones: first, because an evenly, finely roughened surface holds a lot of ink, allowing deep solid colors to be printed; secondly because the process of smoothing the texture with burin, burnisher and scraper allows fine gradations in tone to be developed.

The mezzotint printmaking method was invented by Ludwig von Siegen (1609–1680). The process was used widely in England from the mid-eighteenth century, to reproduce oil paintings and portraits.
http://en.wikipedia.org/wiki/Printmaking