Just to share my ideas..... These Nephilim were the offspring of Cain, they were the "serpent seed" but still subject to be redeemed because they became humans or they got the chance to become humans. They were the 'spirit in prisons" visited by Christ during 3 days, and the Gospel was ALSO preached to them(see 1 Peter 4:6 and 1 Peter 3:18–20) . Of course, if the Gospel being preached it's attached salvation.
I have an answer but I would like for you to tell me your answer first.
Jud 1:6 And the angels which kept not their first estate, (gave up mortality for immortality" but left their own habitation, (left their habitation in heaven for habitation on earth) he hath reserved in everlasting chains under darkness unto the judgment of the great day.
The fallen angels spoken of in
2Pe 2:4 For if God spared not the angels that sinned, but cast them down to hell, and delivered them into chains of darkness, to be reserved unto judgment;
Which is why the Word of God calls the fallen angels "The sons of God". The giants of Genesis were fallen angels.
Gen 6:2 That the sons of God saw the daughters of men that they were fair; and they took them wives of all which they chose.
Proof that the Word of God uses the term "sons of God" to speak of angels comes from the book of Job where the term 'sons of God" is used twice more to describe angels.
Job 1:6 Now there was a day when the sons of God came to present themselves before the LORD, and Satan came also among them.
Job 2:1 Again there was a day when the sons of God came to present themselves before the LORD, and Satan came also among them to present himself before the LORD.
This whole idea or teaching that some how the giants of Genesis 6 were the off spring of Cain or Seth is nothing but garbage and there is no scripture to prove the giant were anything but fallen angels which can be proven with the scriptures I just quoted.