Here "gate" is a metaphor that means Jesus provides access into the sheepfold where God's people dwell in his presence. AT: "I am like the gate that the sheep use to enter into the sheepfold" (See: [[rc://en/ta/man/translate/figs-metaphor]])
This refers to other teachers who have taught the people, including the Pharisees and other Jewish leaders. AT: "All of the teachers who came without my authority" (See: [[rc://en/ta/man/translate/figs-explicit]])
These words are metaphors. Jesus calls those teachers "a thief and a robber" because their teachings were false, and they were trying to lead God's people while not understanding the truth. As a result, they deceived the people. (See: [[rc://en/ta/man/translate/figs-metaphor]])