It sounds like you have a Porcelian Fused to Metal crown on the tooth and the gum receded leaving the gray area. The best solution would be to have the crown replaced with an all porcelain crown to precent the dark gray area from being visible. Talk to a local dentist about your options
Frequently patients that haves crowns and bridges made with a metalic core (Porcelain Fused to Metal Crown and Bridges - PFM) suffer from gum receding, thats very common.
The solution is to have them replaced for crowns metal free (In-ceram, E-Max, Bruxzir) that will solve the black lines near the gum.
if the reason your gum has receded is because there is decay around the margin of the crown, you need to have the crown replaced. if it is a gum issue and there is no decay, just recession around an otherwise healthy alive tooth, you maybe able to have a gum-graft to replace the lost soft tissue/gum.