While I have been reading some of the comments, it seems that before I even try to refute the argument, some clarification needs to be made.
First of all, God doesn't send people to hell for what they don't know but for what they know and choose to ignore. In Romans is explained that God gave us a conscious so that we intrinsically know good from evil and we (ALL) are without excuse.
Secondly, it also says that we (ALL) know the God that exist but we suppress the truth because we love our sin.
So laying that out, whether you hear the Gospel or not is irrelevant when it comes to justifying hell .
On the other hand, justification (salvation) does come by believing the Gospel. So one may make the argument that practically speaking is the same thing but it isn't.
The CRIME is not that they don't BELIEVE but that they do KNOW but CHOOSE to ignore what they know.
So what about that tribe in the middle of the Amazon that never heard? The Bible answers this objection by say that he (God) created mankind from one blood and appointed times and places for them so that they may seek God. So depending from what theological branch of Christianity you come from you may conclude that;
1 The people that never heard wouldn't have believe anyway (by believe we Christians mean repent and truth in Christ as oppose to just accepting an intellectual or philosophical view).
2 God justly decided to pass on them because they do deserve their punishment and he doesn't owe them any mercy and it is already a great thing that he has chosen to save some out of love.
If you like these sort of conversations you should checkout this book. https://www.amazon.com/dp/B004YE7O3E/ref=cm_sw_r_cp_apa_i_2wZRDbQY8ZZ7A
Or this one if you don't have a very deep background in theology. https://www.amazon.com/dp/B07TZFGNTZ/ref=cm_sw_r_cp_apa_i_vg6iDb25DJWYJ