I have a GTX 1070ti with the latest driver (419.something) and my monitor is a HP OMEN 25 Freesync @ 144hz. I enabled G-Sync for it but sometimes I check NVIDIA Control Panel and in some cases the G-Sync option is not showing. What could be the issue?
It could be an issue with the Nvidia software. Only some Freesync monitors are not available for G-sync, and they might not have the ability to detect and allow for monitors, in a reliable manner yet. I'd post a bug report to Nvidia.