In 'The Old Man and the Sea', the old man's own mental state can contribute to seeing it as a Gothic novel. He endures great physical and mental strain. His struggle is not just physical but also psychological. This internal struggle in the face of a harsh external environment is somewhat Gothic, as Gothic novels often explore the inner turmoil of the characters along with their external challenges.
The power of nature in 'The Old Man and the Sea' gives it a Gothic feel. The huge fish and the sharks that attack are like menacing forces that are beyond the old man's full control. This is similar to how in Gothic novels, there are often powerful and uncontrollable forces that the protagonist has to face, be it a supernatural entity or a powerful tyrant.