And what did Nimrod do exactly that was making him a Gibborim?
Who knows? But the language that he began to be a “mighty one” links up with the description of the Nephilim as “mighty.”
Did he access pre-Flood knowledge?
Did he engage in an act of cannibalism on some Nephilim remains? See this article:
https://www.newscientist.com/article/dn18172-gene-change-in-cannibals-reveals-evolution-in-action/
The issue with this thought (Nimrod) was that he was the son of Cush, not Canaan.
Also, Genesis 6:4 seems pretty clear. Could be there was a second violation.