If there is no h/o trauma(which can cause , the most probable cause would be food deposition chronically due to improper brushing in few surfaces of the particular teeth, which caused hard calculi formation. That eventually pushes the gum off the teeth with time gradually, if it's neglected.
Next Steps
get scaling done as soon as dentist is available.
followed by that need to look into proper brushing technique to clean thoroughly all the surfaces of teeth
use dental floss regularly
tongue cleaning regularly with tongue scraper
mouthwash : Betadine mouth gargle, once in every 2 days, after food. and after holding mouthwash in mouth for 2-4mins. do not rinse your mouth or eat/drink for next half an hour.
Luke warm water salt gargle(if blood pressure within normal limits), or no salt to be used, followed by scaling.