That's simply referring to the adaptive sync implementation, which prevents screen tearing without introducing significant latency by adjusting the screen's refresh rate to your frame rate. "G-Sync" monitors use a proprietary chipset from nvidia to implement the feature. "G-Sync compatible" monitors are DisplayPort Adaptive Sync (aka FreeSync) monitors that Nvidia has verified should meet their criteria for a G-Sync display. The feature can now be enabled for other monitors that support FreeSync over a DisplayPort connection as well, but Nvidia can't guarantee that those screens meet the same criteria as ones that they classify as G-Sync Compatible.
Personally, I would prefer a G-Sync Compatible (FreeSync) display over a G-Sync one, simply because DisplayPort Adaptive Sync is an industry standard that is now supported by both Nvidia and AMD, and probably Intel in the near future, whereas regular G-sync will only likely ever be supported by Nvidia, which could limit one's options for graphics cards that support the feature if the competition ends up offering better hardware a couple years down the line. And of course, G-Sync screens typically cost notably more for essentially the same thing.