https://www.reddit.com/r/hardware/comments/998zkw/freesync_on_an_nvidia_gpu_through_an_amd_gpu/
tl;dr:
Who wants to bet how long it'll be until nVidia either (a) find a way to disable this workaround, or (b) announce Adaptive Sync support by complete coincidence?
tl;dr:
- Add AMD GPU to system and plug monitor into it;
- Find a game that allows you choose a rendering device;
- Choose nVidia card as rendering device;
- FreeSync works as expected via AMD GPU.
Who wants to bet how long it'll be until nVidia either (a) find a way to disable this workaround, or (b) announce Adaptive Sync support by complete coincidence?

Maybe am wrong on thinking nvidia was the first to bring out variable refresh rate (Please correct me if am wrong)