It is true that humans saw a great change in their gnomes after the Flood. Prior to the Flood, gnomic behavior was much like that of humans—violent, wicked, every inclination of their hearts evil all the time. Noah found them particularly troubling. When they heard he could only take two on the ark, they hid his tools, spilled his boiling kettles of pitch, and kidnapped the unicorns. The ark would have been completed much more quickly if Noah's sons hadn't needed to take shifts watching out for gnomes slipping in two male lions and replacing the gopher wood with real gophers.
The gnomes didn't waste the gopher wood, though. They used it to build beer barrels. When the Flood came, they quickly drank all the beer, hopped into their barrels, secured the lids, and held on for dear life. With twelve gnomes per barrel, holding on wasn't too difficult.
When the flood waters receded, the barrels had drifted all over the earth; no two barrels landed in the same location. Those who landed in Africa became Abatwa—tiny hunters who live in the grass and ride ants. Those who landed in Hawaii became Menehune—the fish herders. Others became house elves, Hobbits, Santa's helpers, Pixies, Leprechauns, Yule Lads, and the like.
It is true that the post-Flood gnomes are significantly less violent and troublesome than their predecessors. Sure, there are still sprites who lead their human victims into the bog, but overall, they have changed. It's hard to say, however, whether God supernaturally tempered their personalities or if the trauma of riding out the Flood in barrels and being separated from their kinfolk made them more cautious.
Humans saw a different kind of change. After the Flood,
lifespans decreased quickly and significantly. We don't know if it was due to environmental conditions after the Flood or if God tampered with the human genome.