If you have not noticed yet the P6 series now have two GPU modes:
-MS HYBRID - Just like NVIDIA Optimus in this mode your laptop will automatically switch between INTEL and NVIDIA GPU. This is ideal for longer battery life.
You will basically be using the Intel HD graphics until you open a game or a 3D hardware accelerated app, when you open anything 3D the NVIDIA will kick in - YO!
-DISCRETE- No more switching, IntelHD will be gone and you will ALWAYS be using the NVIDIA GPU. - Notice that this is the required mode to use G-SYNC
To choose modes you can just open the CONTROL CENTER app (in your tray icons) and go to the GAMING tab.
After changing this you always need to restart, and the first time you do it you might even need to restart twice :O
After switching to DISCRETE you can now open the NVIDIA panel and activate G-SYNC.