G-Sync on with V-Sync off and a framerate cap at 144 FPS might still tear depending on how the framerate timing is done through a mixture of number rounding precision and that you might get some frames really quick one after another that are less than 6.944444ms apart if it doesn't enforce a minimum timeslice between frames. This is one of the reasons people used to cap at 135 FPS in games like CS (same engine) as it gives enough buffer you won't occasionally get frames tearing as well as optimal latency.
With G-Sync on and V-Sync off and no framerate cap rendering very quickly will still get some tearing frames but very high framerates the tearing tends to be less noticeable unless you find a scene with lots of parallel lines like blinds or radiators, etc.
V-Sync settings other than the one in the nVidia control panel can also produce unpredictable results with G-Sync and should be off in the games themselves. Largely shouldn't be a problem but some games do odd things internally with their built in V-Sync option.
EDIT: This is one of those situations where it is worth trying G-Sync on and FastSync on - as it should alleviate any tearing while allowing you to render very fast though you might have to experiment with a framerate cap to avoid stuttering - often 180, 200, 240 or 288 fps can work sometimes you need to try values around those rather than precisely those.
