Is the belief “many are saved having never heard the gospel” biblical?
No. But something I would bring up in this particular discussion is that we need to understand that "The Gospel" is a much-confused term for most. First, we need to understand that when Scripture speaks of "The Gospel" we have to keep it in its proper context. While "The Gospel" has a Biblical overview as a reference to "The Gospel of Jesus Christ," we have to also keep in mind that understanding, just as Revelation itself—was progressive. So while the Proto Evangelium may have told Adam that the seed of the woman would destroy the enemy of Man, that doesn't mean that we forget that the Gospel of Jesus Christ was a mystery, and remained unrevealed until the coming of the Comforter at Pentecost.
Consider:
Colossians 1:25-27 King James Version
25 Whereof I am made a minister, according to the dispensation of God which is given to me for you, to fulfil the word of God;
26 Even
the mystery which hath been hid from ages and from generations, but now is made manifest to his saints:
27 To whom God would make known what is the riches of the glory of this mystery among the Gentiles; which is Christ in you, the hope of glory:
Here we see just one of Paul's statements that The Gospel of Jesus Christ was hid from "ages and generations," but is now made manifest. It is in this Age tat God has made known to us the "riches of the glory of this mystery," which is the indwelling of the Lord Jesus Christ. This was not known to the Old Testament Saints.
So let's look at the context of "The Gospel" here:
Galatians 3 KJV
8 And the scripture, foreseeing that God would justify the heathen through faith,
preached before the gospel unto Abraham,
saying, In thee shall all nations be blessed.
"The Gospel" given to Abraham was that the world would be blessed through his seed (offspring). But what was not revealed to Abraham, nor any of his sons and daughters until Pentecost—was exactly what that would mean.
Likewise, Isaiah gave brutal detail of the Death of Jesus Christ, but what does Peter do when the Lord Himself gives him the Gospel?
Matthew 16 KJV
20 Then charged he his disciples that they should tell no man that he was Jesus the Christ.
21
From that time forth began Jesus to shew unto his disciples, how that he must go unto Jerusalem, and suffer many things of the elders and chief priests and scribes, and be killed, and be raised again the third day.
22
Then Peter took him, and began to rebuke him, saying, Be it far from thee, Lord: this shall not be unto thee.
23 But he turned, and said unto Peter, Get thee behind me, Satan: thou art an offence unto me: for thou savourest not the things that be of God, but those that be of men.
This is not a moment of weakness for Peter, it is simply a matter that the Gospel of Jesus Christ was not yet revealed. Again, it would not be until the Comforter came at Pentecost and men were at that time baptized into Christ, receiving the Eternal Indwelling of God, and themselves began preaching the mystery of the Gospel.
The "Gospel" Abraham heard was not—Jesus Christ is the Son of God Who has died in your stead that you might be reconciled to God. It was ...
Galatians 3 KJV
8 And the scripture, foreseeing that God would justify the heathen through faith,
preached before the gospel unto Abraham,
saying, In thee shall all nations be blessed.
You and I, as born again believers, and privy to the Gospel of Christ as revealed to us by God, can understand what the blessing that is given unto all nations is. We can understand what Isaiah meant when describing Christ's death.
But they could not. Understanding was withheld from them until that time when God would make it known unto men.
So in answer to your question, no, a man cannot be saved apart from the Gospel, because only the Gospel can save men. It doesn't matter whether that Gospel is in an Old Testament context, or a New—faith in the revealed will of God is the only means of salvation. There is no salvation for men by "being good," because it is not possible for men to be righteous before God and therefore independent of His grace and mercy. There is none righteous—no, not one!
If you look at the Old Testament Saints, you are going to see one thing they all have in common: faith in the revealed will of God. While I do believe that God reveals His will to all men, and that sometimes that is directly from God and apart from Scripture (Romans 1:19-20; Romans 2:14-15), I don't think this is something happening that often (at this point in time) in this Age. The Word of God has traversed the earth and most have heard about the Lord at this point. Even if this happens, we can still see that what God imparts to the heart of a man—is the Gospel.
Is it “another gospel” as condemned by the Apostle Paul in Galatians?
peace to you
I would say yes, it is. The testimony of Jesus Christ is the spirit—it is the life—of prophecy. Prophecy has a central theme the coming of Jesus Christ for the salvation of Man. The idea that men can be saved apart from the Gospel, regardless of the Age, regardless of the progression of Revelation (the giving of knowledge, not the Book (though that is certainly relevant, lol))—is absurd.
God bless.