Explanation:

- He is: This part is grammatically correct. It's a simple subject-verb structure.

- The same man: This segment is properly structured and identifies a specific person.

- Which I saw: Here lies the error. The word "which" is used incorrectly as it should refer to things, not people. Use "whom" or "that" instead. So, it should be "whom I saw" or "that I saw."

- At the gate: This is a prepositional phrase that correctly describes the location.