Upon what rock?
The rock Jesus is referring to is Peter's confession that Jesus is the Christ, the Son of the Living God.
The church is built upon that single confession - Jesus is the Christ. Jesus is the Son of God.
And on this simple confession, the gates of hell are broken.
The church has always been in the business of taking hell out of business.
Wherever the church of Jesus Christ is, the gates of hell are to be torn down, the influence of hell pushed back, the tormented messed up lives of hell redeemed.
That's who the church is.