I seem to have been able to get Freesync with a 6000 AMD GPU working over HDMI on the LG B9 at 4k 120hz by using CRU (Custom Resolution Utility) and modifying the display. Some junk resolution support was removed to provide enough bits on the extension block to add Freesync Support. It took me a long time to figure out exactly how to get it working because, my particular panel fails to output with a freesync range of 40 to 120hz. It does however work when set to a range of 40-119hz. This passed my validation tests, some gaming and the use of the UFO VRR test. - I will update if I have any trouble.

Some other users seem to have reported having it work with a 40-120hz range so it might be that simple for and AMD 6000 users looking to get Freesync working on these displays, and perhaps other similar displays from LG.

The card I was using was a reference design 6900XT.