It's far from practical, and it's annoying that it's even necessary. But if you find yourself in a situation where you have a Nvidia GPU and only a Freesync monitor, there is a way to get the two talking.
As a way to poke fun at Nvidia for not supporting the Freesync open standard, Linus Tech Tips ran a little test. They grabbed one of the Ryzen 2400G APUs, and paired it with a discrete Nvidia GPU.
The Freesync monitor, which looked like a 27" LG screen from what I could see, was then plugged into the onboard graphics in the motherboard. Linus then jumped into the BIOS and set the primary video device to the integrated graphics. Nothing is plugged into the Nvidia GPU.
Because of that, Windows will then display a different option: Graphics Settings. In there, you can add a variety of programs and then select from power saving or high performance modes. Each of the modes is tied to a different GPU, which means you can then nominate the discrete GPU for your games, while still enjoying FreeSync support.
Theoretically, you should be able to do this with Intel's AMD-powered onboard graphics when that becomes available. Linus also tested the opposite solution: getting G-Sync working with an AMD GPU. That functioned as well, although you need a discrete Nvidia GPU installed to make it work.
The main kicker here is that you have to jump into the Windows graphics settings to do this for every single game. That's time consuming and hugely impractical. More importantly, most people won't have the hardware: if you're using a PC that has a discrete GPU, and you've spent a good amount of money on it, chances are you're running a much higher end Intel or AMD CPU. AMD's higher end Ryzen CPUs don't have integrated graphics at all, and the Intel onboard graphics won't work with G-Sync.
But that's really not the point. The point here is to show that there isn't a physical limitation that prevents Nvidia cards from playing nicely with FreeSync monitors. Nvidia would argue that their solution is better, but how many gamers can see the difference? There are also some situations where Freesync is the only option available, either due to stock, or the display size: Samsung's TVs have Freesync support, but Nvidia's G-Sync enabled large format displays won't come to Australia any time this year.
It's also just cool to see people work their way around restrictions, regardless of how impractical the actual solution is.