Fortnite does Support NVidia Reflex
If you want to understand what is latency and how it can be improved, give this a read:
Introducing NVIDIA Reflex: Optimise and Measure Latency in Competitive Games
Blurbusters made a test on input lag and G-Sync
G-SYNC 101: Optimal G-SYNC Settings & Conclusion | Blur Busters
On their last page they have a FAQ
G-SYNC 101: Closing FAQ | Blur Busters
TLDR:
Turn on G-Sync, use Fullscreen-Mode, Activate NVidia Reflex