I believe G-SYNC is a long term investment for NVIDIA. NVIDIA has a pretty strong percentage of the graphics card market right now, it's not unreasonable to assume they could succeed in it. The benefit is that GSYNC is here now, and more and more monitors are going to come out with support for it.
Making it part of DirectX is completely unnecessary. It has nothing to do with DirectX. DirectX is for rendering graphics on the GPU side. G-SYNC is for receiving frames on the monitor side from the GPU and displaying them as soon as they are available, instead of your typical monitor that just displays the image when the next refresh cycle comes. Likewise, FreeSync will also have nothing to do with DX12 for the same reason
Keep in mind, FreeSync and Adaptive Sync are NOT the same. If you read AMD's FAQ on FreeSync, it is clear that FreeSync is AMD proprietary as well (you need an AMD card to use FreeSync drivers) and is only useful for monitors that implement the Adaptive Sync standard that comes with DP 1.2a. Even if NVIDIA decides to adopt the 1.2a DP standard, they would still have to come up with their own solution for utilizing Adaptive Sync (which STILL requires different hardware on the monitor side, so not every manufacturer is going to do this) so it's unlikely they are going to stray from their first-to-market solution anytime soon. On top of that, GSYNC monitors are already here(sort of). Adaptive Sync monitors that comply with the DP1.2a standard, and thus are then FreeSync ready, probably won't start showing up till Q4 2014 or Q1 2015.