On the other hand, for some reason it is very hard to believe that ALL THREE independent sources are lying at once. Yes, and photos of the original GIGABYTE CDs for these video cards, on which support for 9x is clearly indicated, makes you somewhat doubtful. Unfortunately, we won't find out the truth until we find this disk.
Here are more examples of photos of such CDs for Gigabyte 6-series video cards, where the presence of a driver for Windows 98 is explicitly indicated:
128Mb <PCI-E> DDR2 GIGABYTE GV-NX65128D (OEM)64bit D-Sub+DVI+TV Out <GeForce 6500>
https://www.nix.ru/autocatalog/gigabyte/video … 3.html#pid=2247
https://static.nix.ru/images/gigabyte-gv-nx65 … ft&view_id=2247
256Mb <AGP> DDR2 GIGABYTE GV-N62256DP2-RH (OEM) 64bit DVI+TV Out <GeForce 6200>
https://www.nix.ru/autocatalog/gigabyte/video … 9.html#pid=2247
https://static.nix.ru/images/gigabyte-gv-n662 … ft&view_id=2247
256Mb <PCI-E> DDR2 GIGABYTE GV-NX66T256DE (RTL) D-Sub+DVI+TV Out+SLI <GeForce 6600GT>
https://www.nix.ru/autocatalog/gigabyte/video … 7.html#pid=2247
https://static.nix.ru/images/gigabyte-gv-nx66 … ft&view_id=2247
256Mb <PCI-E> GDDR3 GIGABYTE GV-NX68G256D-B (OEM) D-Sub+DVI+TV In/Out+SLI <GeForce 6800GS>
https://www.nix.ru/autocatalog/gigabyte/video … 7.html#pid=2247
https://static.nix.ru/images/gigabyte-gv-nx68 … ft&view_id=2247
It turns out that even on later revisions of disks (Up to version 4.1), they kept a mention of Windows 9x support. And this is most likely not an error. And although none of these disks can be found on the Internet anymore, the first one is still interesting (For GeForce 6500\6200LE), because there is a high probability that the driver you are looking for is there.
Maybe there is some website on the Internet where collectors of old video cards live, from whom you could ask about the presence of such CDs in their extensive collections?