Implicit in such an assumption is that God has said, "I will give you an explicit opportunity to accept the Gospel." But I don't see where God has said that. Now, perhaps He has and revelation has simply not told us.
And the bolded portion and the italicized "PERHAPS" is precisely where we are forced to leave it. We can't go beyond what Scripture tells us. But we CAN speculate based upon the following:
1) God's loving and fair character and His own words that state He "desires all people to be saved and to come to the knowledge of the truth," and that declare, "I have no pleasure in the death of the wicked, but that the wicked turn from his way and live" - these show a God Whom has a heart and desire that men come to Him and be saved. His heart for the salvation of men is clear per the enormous suffering Jesus endured during His ordeal of the Cross.
2) We see that Christ leads by example - does He expect us to love others as we love ourselves, but yet He Himself shows only hate and condemnation without giving people a chance? Does He expect us to be more moral (per His definitions of that ) than He is? Absurd! His parables: Deny the lawyer trying to choose for himself who his neighbor is, who he is to treat with kindness and love; Show great joy in the return of the Prodigal Son. The Samaritan's treatment of the robbed and beaten man is held up as how God want us to treat others - to show compassion to ANYONE in need. How could God be any different? Are we to show more compassion than God? A man without understanding of God is a man in the worst kind of need - would a loving God withhold knowledge that would save a man (the Gospel) if He foreknew that man's heart would be receptive to His message? I don't believe that. And this tells me that something else is going on - that God foreknows many will only reject God FURTHER, as they have already rejected. OR perhaps God has long reached to many miraculously, in ways that we have not seen. In fact, that would not surprise me a bit.
3) Does the Father send Jesus to come to save "the world" and then deliberately bypass or place others in time and place so that they can never know what He requires of them to respond to (the Gospel message) - or is the Gospel withheld via their physical and time placements because He foreknows all for whom knowledge of the Gospel will only be a more specific revealing of God that they will only reject as well?
4) If rejection of the Gospel is a rebellion and unsaved people are undoubtedly rebels, does God deliberately guarantee life-long rebels by giving them a requirement (having faith in Him - and post Jesus - belief in the Son and His Resurrection) and then deliberately withhold the information (the Gospel) that would give people an opportunity to positively respond as God requires, UNLESS there is something more going on?
5) People can, do and long historically have rejected God without knowing about Jesus. You cannot divvy God up - if you reject the Father, the Son or the Holy Spirit, you have rejected ALL of Who God is. So one who merely denies God with only very basic knowledge of Him has nonetheless also rejected Jesus - it's just that he doesn't know that is what he has ALSO done. And just look at all those whom actually saw Jesus in the flesh, doing incredible wonders, yet STILL would not believe. So we often falsely believe that people have an information deficit. But that is not what Scripture reveals.
6) When Scripture says: "All who sin apart from the law will also perish apart from the law, and all who sin under the law will be judged by the law. 13 For it is not those who hear the law who are righteous in God’s sight, but it is those who obey the law who will be declared righteous. 14 (Indeed, when Gentiles, who do not have the law, do by nature things required by the law, they are a law for themselves, even though they do not have the law. 15 They show that the requirements of the law are written on their hearts, their consciences also bearing witness, and their thoughts sometimes accusing them and at other times even defending them.) 16 This will take place on the day when God judges people’s secrets through Jesus Christ, as my gospel declares" - we see that God sees things we do not about people's hearts and minds, and we also see that He has put His law on their hearts. This tells me God has not withheld knowledge of Himself that - IF embraced and desirous of MORE knowledge - God would gladly give it (the Gospel, etc). And, interestingly, before we see this section as implying that God hasn't treated everyone with equal consideration, He leads off with this declaration: "For God does not show favoritism." Note that God didn't limit those He desired to His original chosen people - as, all the time, He had a plain to save Gentiles as well." Note that heaven will include those from every tribe.
7) In heaven, John saw, "a great multitude that no one could number, from every nation, from all tribes and peoples and languages, standing before the throne and before the Lamb, clothed in white robes, with palm branches in their hands." And so how could that be - it may well be more than because of children who died (I believe there is an implied "age of accountability"). Yes, God very likely has communicated the Gospel to many in ways we cannot see and know of.
Given God's Holy, just and loving character, I just can't imagine that His plan of salvation isn't designed for all - across the entire world and all of its ages - to save the maximum possible number of people who would respond to the Him with a saving faith. The idea that God has played favorites and kept knowledge (key to their salvation) from them so as to prevent their salvation, or has designed them incapable of knowing or desiring Him - I find those things absurd.
9) I believe it is the devil whom wants us to believe that God is stingy with knowledge about himself, or that He desires anyone to reject Him, or that He has PREVENTED any one from coming to Him.
Again, we can only go on what Scripture says, but we can speculate. And Scripture shows no one being saved (post Jesus) that did not first hear the Gospel!