I hate it when there are a lot of people.
In the sentence “사람 많은 게 싫어요”, 게 is a noun-forming particle that turns the adjective 많은 (many) into a noun meaning “the state of being many.” The noun-forming particle 게 can also be used with other adjectives or verbs to create nouns that represent the state, condition, or action described by the adjective or verb. For example:
어두운 게 무서워요. (The state of being dark is scary.) 높은 게 보기 좋아요. (The state of being high is pleasing to see.) 춤추는 게 좋아요. (The action of dancing is enjoyable.)