Is it OK if my porcelain crown has gap between the crown and gum line at the back? (photo)

It's difficult to tell from the photo you have provided, but it seems that the margins are sealed and that you have a little gum recession on the back of your tooth.  I would go back to your dentist and have them check to make sure the margins of the crown are sealed properly.  If they are, you have nothing to worry about.  If the margins are not sealed, have the crown redone to avoid any future problems.

Hi:As long as the edge of the crown and the tooth is sealed, the gap makes no difference.  In most cases you can keep the margin clean easier when it is visible.  Best,Dr. Maddahi

What you might be seeing is not a gap but the crown not going under the gum on the inside. What you can then see is your tooth structure between the crown edge and the gumline. That is absolutely no problem. If there were a space between the crown and your tooth, that would be a different story. I would go back to your dentist to make sure there is no opening between tooth/crown just to make sure. I hope this helps!