G-Sync Features  
G-Sync monitors typically carry a price premium because they contain the extra hardware needed to support Nvidia’s version of adaptive refresh. When G-Sync was new (Nvidia introduced it in 2013), it would cost you about $200 extra to purchase the G-Sync version of a display, all other features and specs being the same. Today, the gap is closer to $100.
However, FreeSync monitors can be also certified as G-Sync Compatible. The certification can happen retroactively, and it means a monitor can run G-Sync within Nvidia's parameters, despite lacking Nvidia' proprietary scaler hardware. A visit to Nvidia’s website reveals a list of monitors that have been certified to run G-Sync. You can technically run G-Sync on a monitor that's not G-Sync Compatible-certified, but performance is not guaranteed.

There are a few guarantees you get with G-Sync monitors that aren’t always available in their FreeSync counterparts. One is blur-reduction (ULMB) in the form of a backlight strobe. ULMB is Nvidia’s name for this feature; some FreeSync monitors also have it under a different name. While this works in place of Adaptive-Sync, some prefer it, perceiving it to have lower input lag. We haven’t been able to substantiate this in testing. However, when you run at 100 frames per second (fps) or higher, blur is typically a non-issue and input lag is super-low, so you might as well keep things tight with G-Sync engaged.

G-Sync also guarantees that you will never see a frame tear even at the lowest refresh rates. Below 30 Hz, G-Sync monitors double the frame renders (and thereby doubling the refresh rate) to keep them running in the adaptive refresh range.

FreeSync Features
FreeSync has a price advantage over G-Sync because it uses an open-source standard created by VESA, Adaptive-Sync, which is also part of VESA’s DisplayPort spec.
Any DisplayPort interface version 1.2a or higher can support adaptive refresh rates. While a manufacturer may choose not to implement it, the hardware is there already, hence, there’s no additional production cost for the maker to implement FreeSync. FreeSync can also work with HDMI 1.4.  (For help understanding which is best for gaming, see our DisplayPort vs. HDMI analysis.)

Because of its open nature, FreeSync implementation varies widely between monitors. Budget displays will typically get FreeSync and a 60 Hz or greater refresh rate. The most low-priced displays likely won’t get blur-reduction, and the lower limit of the Adaptive-Sync range might be just 48 Hz. However, there are FreeSync (as well as G-Sync) displays that operate at 30 Hz or, according to AMD, even lower.

But FreeSync Adaptive-Sync works just as well as any G-Sync monitor. Pricier FreeSync monitors add blur reduction and Low Framerate Compensation (LFC) to compete better against their G-Sync counterparts.

And, again, you can get G-Sync running on a FreeSync monitor without any Nvidia certification, but performance may falter.