The GL703GM doesn't have G-sync on any of the models as far as I know(?), only the GL703GS does.
The Asus' website doesn't say anything about g-sync either
https://www.asus.com/Laptops/ROG-Strix-SCAR-Edition/Tech-Specs/, retailer probably falsely adverstising the laptop or they messed up the models.
I avoided buying it solely for that reason and got the GS instead.