In the sentence "There is 0 mm of attached gingiva," is the proper word "is" or "are" when asking about the millimeters?
It's plural: "How many millimeters?", and this is even if the number is 0.
There was a similar question but I couldn't find it, where the question was about numbers below 0.
There is 1 millimetre and there are 0 or more millimetres
1 is singular
0 or anything but 1 is plural