It is common to say, "he has a gentle manner". This means he is kind, sympathetic and kind in his actions toward other people. His kind or gentle manner is his behaviour. However, the second sentence is not natural. People might say, "He was IN a gentle mood" or "he is IN a bad mood". The problem is using a preposition. You can be "in a mood", "in a rage" or "in good spirits". These are passing feelings, but not part of a person's personality and not the same as a person's manner. Hope this helps

We never use the latter, as it is incorrect, and seldom use the former, which is old-fashioned. Another option is "His manner was gentle"