Q:  What are the “gates” of Hades?  Aren’t they opposed to Jesus’ church?

A: There are actually two questions.

FIRST QUESTION:  I understand “gates” to be a figure of speech for “power” or “authority.”

Each meaning must be determined from its context, and usually means the doors that can shut off a house or a walled city. But, in a number of cases, such as the verse above, it becomes a metaphor.

“Hades,” in my opinion, means simply, “the unseen,” and is equivalent of “sheol,” the OT word translated mostly as “the grave.”  See Acts 2:27 which quotes Psalms 16:10.

Therefore, I would interpret Matthew16:18 as “...thou art Peter and upon this rock I will build my church; and the ‘powers of the unseen’ shall not prevail against it.” 

Some might prefer ‘powers of the grave,’ knowing from Jesus’ words that even though they die, death cannot hold his church (ecclesia), for they will be resurrected to life.

I think of ‘powers of the unseen’ to include all the unknown powers of this world system, as well as the unseen world of Satan and all his evil horde.

SECOND QUESTION:  Perhaps this has been answered.  Anyway, yes death, plus the unseen powers are opposed to Jesus church. Death is an enemy, but Jesus promise was to believers then, that even though they die, he would raise them up at the last day, and they would enter the kingdom.