Do gums grow back after veneers?

That's a hard question to answer as it is both yes and no.  In many cases, the provisionals put pressure on the gum tissue between the teeth and that takes several weeks to months to settle back in...  on the other hand, if the veneers weren't prepared or built to close that space, the tissue won't ever fill in completely.  Probably the best way to monitor this over the next few weeks would be to take a photo of the black triangle and then take another one in a few weeks and compare them.  If the gums haven't changed at all over the next month, then they should be replaced if you want to close the space.  
For sure make sure that the dentist is in the loop.  He/she may know that the tissue won't fill in already and simply suggest that you remove and replace them with new veneers.  It may take a slight change in the way it is prepared, and it may just be a matter of the lab building the contact between the teeth a little closer to the gums in the first place.  If you do get them replaced, make sure the temporary is clean between the teeth so that gum tissue isn't irritated on this round...
In our practice, we would do this all at no charge as it is already frustrating for everyone, but the solution wasn't what you were after.  Calm and clear conversation with the dentist should help to find a relatively simple solution...