From what I understand so far:
1. You need GTX1xxx or newer to have GSync working on FreeSync monitor.
2. You need Win10.
3. Many or most FreeSync monitors outside from NVidia's official list should work.
4. There is still no better list, which monitors are working, aside those few monitors NVidia mentioned during CES 2019.
5. My feeling says, if your Freesync monitor can run 144Hz or faster, it should work.
I dunno if similar idea about the list has been put to reality somewhere in the internet.
If someone from Tomshardware is reading this thread, perhaps it is a good idea to make/moderate a sticky thread about GSync monitor compatibility list. Forum members will then contribute to fill in the list, which FreeSync monitors are compatible. The list would be very helpful for people, who already have FreeSync monitor+nVidia card combination or who want to safe some money by buying FreeSync monitors rather than native GSync monitors.