The Septuagint translated "Nephelim" as "Gigantes"....
You need to justify that statement with some serious research.
While I do think that "Nephelim" and "Gigantes" are not perfect equivalents...(often true with any language translation). The translators at the time simply disagreed with you.
They spoke the Greek of the day, and they were far less removed from Biblical Hebrew as we are...
And, they disagree with you.
Since you have obvious in-depth knowledge of their perfidy in mistranslating, please enlighten us.
Interpreting Genesis 6:1-2 as being an intermarriage of human women with fallen angels provides the only adequate explanation for the statements found in II Peter and Jude. In these passages, three points are made. First, this phenomenon of intermarriage was a peculiar, unique sin; second, the sin was timed in conjunction with the Flood; and third, it is different from the original fall of angels; otherwise, all angels would end up being in permanent confinement. (See discussion that follows on this point.) The first passage, II Peter 2:4-5, talks about a confinement of a select group of angels. Verse 4 mentions the place of confinement as being Tartarus, and Tartarus must be distinguished from the Abyss. The Abyss is a section of Sheol or Hades, which is a temporary place of confinement for fallen angels. Tartarus is another section of Sheol or Hades, which is a permanent place of confinement for fallen angels. Tartarus is described as being pits of darkness, and they are reserved unto judgment. These angels confined in Tartarus are reserved unto the Great White Throne Judgment. Unlike the demons in the Abyss that do get released after the course of time, those in the Tartarus will never be freed. They will go directly from Tartarus to the Lake of Fire, by means of the Great White Throne Judgment. The confinement of these angels permanently was necessary so as to assure that they would not repeat the sin again with subsequent women after the Flood. Then verse 5 reports on the timing of their confinement, and the timing is in conjunction with the Flood. The second passage is Jude 6-7. Verse 6 deals with the fact of the fall and makes four statements. First, the rebellious angels kept not their own principality, meaning they did not remain in their position and place of rank in the satanic cosmos. Second, they left their proper habitation, meaning they left the demonic angelic sphere and then entered the human sphere by intermarriage. Third, they are now kept in everlasting bonds under darkness, and they are kept there in Tartarus as a result of the above two sins. Fourth, they are being kept there until the judgment of the great day, that is, the Great White Throne Judgment. Again, the angels who perpetrated this intermarriage will never be loosed at any time to roam free again. Then verse 7 reveals the nature of this sin. The key phrase is in like manner. In like manner as Sodom and Gomorrah, they went after strange flesh. The phrase in like manner with Sodom and Gomorrah means the sin of these angels was also a sexual sin, and the term strange flesh refers to sexual relations that are unnatural. In the case of Sodom and Gomorrah, it was homosexuality; in the case of angels, it was seeking female flesh. Therefore, instead of remaining in their usual state of residence, they made a new state of residence of alien flesh to commit gross sexual immorality. Only by interpreting Genesis 6 to mean angels in keeping with normal usage of the Hebrew terminology can the two passages of II Peter and Jude be understood.
From Fruchtenbaum Genesis Commentary