Adam Clarke on Genesis 22:17

Shall possess the gate of his enemies - Instead of gate the Septuagint have πολεις, cities; but as there is a very near resemblance between πολεις, cities, and πυλας, gates, the latter might have been the original reading in the Septuagint, though none of the MSS. now acknowledge it. By the gates may be meant all the strength, whether troops, counsels, or fortified cities of their enemies. So Mat 16:18 : On this rock I will build my Church, and the gates of hell shall not prevail against it - the counsels, stratagems, and powers of darkness shall not be able to prevail against or overthrow the true Church of Christ; and possibly our Lord had this promise to Abraham and his spiritual posterity in view, when he spoke these words.