Gukesh's last name is Dommaraju. It's his family surname. He is a Telugu person by birth, but he grew up in Chennai, Tamil Nadu. In the state of Tamil Nadu, people often take their father's given names as their last names, and always write it in abbreviation. Indian last names often disclose caste, and due to a widely influential movement in TN (see [0]), most people of TN gave up using caste-based surnames, and switched to solely using father's names. But, the father's name is often written as the first letter of that name, and the person is called like that in official places, too. Among friends, colleagues, teachers, etc., only the given name ever is used.
As Gukesh grew up in Chennai, he used his last name like that. His parents also use one name only.
Anecdote: my distant cousin, a Bengali, also grew up in TN. His parents also Tamilized his name. His name was, say, Rama Dass, and he went by and put his name as D. Rama, or Rama D.
When their family moved back to Bengal, his name was Rama Dass again.
Srinivasa Ramanujan's given name was Ramanujan, and Srinivasa was his father's name.
[0]: https://en.wikipedia.org/wiki/Periyar