G-Sync is **not required**
G-Sync has a significant impact from 30 fps on up ... the impact of that improvement however is harder to recognize the further you get past 60 fps.
FreeSync has a significant impact from 40 fps on up ... the impact of that improvement however is harder to recognize the further you get past 60 fps.
G-Sync allows you to switch to ULMB mode (motion blur reduction) which starts to make sense as an option at about 70 fps. Freesync has no inherent MBR technology. However, you can also utilize MBR technology using toasty strobelight utility
http://www.blurbusters.com/
You can see what monitors support what here:
http://www.blurbusters.com/faq/120hz-monitors/