WHen to use g-sync

[Dz1on1s]

I have 279q rog swift and playing overwatch at 155fps. at ultra settings. Do I use g-sync?

Also when I play witcher I wont get 144fps . Do I need to downgrade my hz of monitor to use g-sync?

 

[Dz1on1s]

When Do I need to use g-sync? Do I use it with in game V-sync?
Playing witcher at max I get 100 fps . Do I need to downgrade hz of monitor to 100?