Dental Bone Graft with non adherent gum tissue 3 weeks out. (photo)

from the picture you have provided it seems that you have a membrane covering the bone graft. The type of this membeane seems to be Gortex ( cytoplast)  which will be removed after 21 days. This membrane could be left exposed ,  and it is designed so that new gum tissue grows underneath it to cover the bone.   Once it is removed , the flaps of gum that you are concerned about not attaching , will close up. So your healing looks normal. If you are still concerned book a follow up appointment to see your surgeon 

As long as there are no signs of infection, this could be just an exposed membrane barrier that was used to contain the bone graft or it could be actual bone particles that are above the healing vascularized area of bone. Please check with your surgeon to confirm.