Gavin often gets punched by Tyler, the school bully. Gavin is also afraid of Tyler's twin brother, Tristan, since Tristan looks just like Tyler. This is an example of:

The given situation is an example of similarity-based fear generalization.

Explanation:

In psychology, generalization is defined as the tendency of the brain to respond in the same way to a similar stimuli.

Humans and animals use this concept to use their past experiences to learn and recognize the similarities between two similar situations.

Fear generalization is a type of generalization in which an individual associates fear experienced in the past to similar situations, people, place, objects etc.

Therefore, Gavin is generalizing and associating his fear of Tyler with Tristan.