Jesus was saying that he was fulfilling that prophecy by his actions and speech at that very time. This can be stated in active form. AT: "I am fulfilling what this scripture said right now as you are listening to me" (See: [[rc://en/ta/man/translate/figs-activepassive]])
"surprised about the gracious things that he was saying." Here "gracious" may refer to 1) how well or how persuasively Jesus spoke, or 2) that Jesus spoke words about God's grace.
People thought that Joseph was Jesus' father. Joseph was not a religious leader, so they were surprised that his son would preach what he did. AT: "This is just Joseph's son!" or "His father is only Joseph!" (See: [[rc://en/ta/man/translate/figs-rquestion]])