Thanks for writing,
Well there are many factors which contribute to a good dental health. Your diet and genetics form an important part of this system.
The tooth which has developed a big hole, needs clinical check up along with a x-ray of the tooth. It can sometimes be due to undermining caries which makes a hole under the surface of your tooth without getting discovered. When the overlying structure becomes fragile enough, it chips off, creating a big hole.
Gum problem is directly related to dental hygiene. i would recommend you to visit a dentist
and have professional cleaning (scaling)done. Use soft bristle tooth brush and be gently on your teeth. Use can also use a mouthwash 2-3 times a day.
Thank you, Dr.Tarush thakur.