You will no doubt be able to discover an art restorer in your area with a simple internet search, however, I would recommend that you first of all research Gainsborough prints on ebay.co.uk. I have just had a look and there are numerous examples for sale including the one you mention. Vintage prints, especially those in poor condition, are often of limited value. You may find that in this case, the restoration costs will exceed the value of the picture. Please do your research before you decide whether to go ahead.