Gap between the dental bridge and gums. Any suggestions?

There should be some space under the bridge so you can properly clean and maintain it. I would doubt that the flakes you saw are zirconia, more likely they were excess cement that was cleaned off the margins but missed from removing completely.  If you have concerns however you should feel free to discuss this with your dentist. 

There should not be any gap, it needs to be touching the gum, passively, but not to have a space, you should see your doctor, to check this , the flakes you are saying, are not normal.