What does the gate symbolize?

The narrow way/broad way motif comes from Matthew 7:13-14: "Enter ye in at the strait gate: for wide is the gate, and broad is the way, that leadeth to destruction, and many there be which go in thereat: Because strait is the gate, and narrow is the way, which leadeth unto life, and few there be that find it." It repeats a number of times, and is most relevant perhaps to John, who metaphorically walks that narrow way in his vision (pg. 231).