I had a conversation with a Church of Christ preacher who believed the gospel was available only to humanity.

I don’t believe this to be the case. See Mark 16:15 KJV. And he said unto them, Go ye into all the world, and preach the gospel to every creature.

Men may not often have the opportunity to preach to every creature or baptize angels, but I believe the gospel of Christ is available to any creature that can understand it. Only Satan, the Anti-Christ, and the False Prophet have received their final judgment. Revelation 10:20

Except for those three, salvation is available to all.