My 5-m.o. Baby keeps on gnawing. Is it a sign that a tooth is about to sprout?

Yes, that is a sign for sure! Parents think of teething as the few days of swelling, discomfort, and irritability before a baby's tooth erupts, but tooth development actually begins before birth. Primary or "baby" teeth form below the gum line around the sixth week of pregnancy, and they're covered by hard enamel during the third to fourth month. Permanent or "adult" teeth also begin developing at this time.