Albert Barnes on Revelation 21:13

On the east three gates, etc. The city was square, (Rev 21:16,) and the same number of gates is assigned to each quarter. There does not appear to be any special significancy in this fact, unless it be to denote that there is access to this city from all quarters of the world, and that they who dwell there will have come from each of the great divisions of the earth; that is, from every land.