What happens to people who never heard the gospel?

Justin Martyr c. 100–c. 165 ADFirst Apology 46.1–4

We have been taught that Christ is the firstborn of God, and we have shown that he is the Word, of whom every race of people has had a share. And those who lived with the Word are Christians, even if they were thought godless, such as, among the Greeks, Socrates and Heraclitus and those like them, and among the barbarians Abraham, Ananias, Azarias, Misael, Elijah and many others.

Justin answers the objection that people born before Christ cannot be blamed. "Word" (logos) also means reason.

Cyprian of Carthage c. 200–258 ADOn the Unity of the Church 6

Whoever is separated from the Church and joined to an adulteress is separated from the Church's promises, and whoever leaves the Church of Christ will not come to Christ's rewards. He is a stranger, he is profane, he is an enemy. He cannot have God for his Father who does not have the Church for his mother. If anyone could escape who was outside Noah's ark, then he too escapes who is outside the Church.

Cyprian is writing about Christians who left the Church in a schism, not about pagans who never heard.

Council of FlorenceCantate Domino (Decree for the Jacobites) 1442

It firmly believes, professes and preaches that no one who does not exist within the Catholic Church, not only pagans, but also Jews or heretics and schismatics, can share in eternal life; but they will go into the eternal fire prepared for the devil and his angels (Matthew 25:41), unless before the end of life they are joined to her.

The decree that received the Copts (Jacobites) at Florence in 1442: the hardest form of 'no salvation outside the Church'. Later Catholic teaching (Pius IX and Vatican II, below) kept the rule and excepted ignorance that is not one's own fault.

Westminster AssemblyWestminster Confession of Faith 10.4

Much less can men, not professing the Christian religion, be saved in any other way whatsoever, be they never so diligent to frame their lives according to the light of nature, and the laws of that religion they do profess. And, to assert and maintain that they may, is very pernicious, and to be detested.

The same section allows that elect infants and others "incapable of being outwardly called" are saved by Christ (10.3).

Pius IXAllocution Singulari quadam 9 December 1854

It must be held by faith that outside the Apostolic Roman Church no one can be saved; that this is the only ark of salvation; that whoever has not entered it will perish in the flood. But it must likewise be held as certain that those who labor in ignorance of the true religion, if that ignorance is invincible, are not bound by any guilt in this matter before the eyes of the Lord.

Both halves in one sentence: the old rule and the exception for "invincible ignorance".

Second Vatican CouncilLumen Gentium 16

Those also can attain to salvation who through no fault of their own do not know the Gospel of Christ or His Church, yet sincerely seek God and moved by grace strive by their deeds to do His will as it is known to them through the dictates of conscience.

The council adds that whatever good is found among them is "a preparation for the Gospel", using Eusebius's phrase.