You can get software from here:
https://www.asus.com/Notebooks_Ultrabooks/ASUS_ROG_G751JM/HelpDesk_Download/The G-SYNC will be available in newer models that specifically say they have it enabled. The community is working on making it work (gamenab is, specifically) since we're probably not gonna get any support from ASUS, at least not officially, for the older models that we have. It's not a hardware difference, though, entirely software-based, so you'll get it one way or the other. The same situation currently exists with MSI, Clevo/Schenker and Gigabyte. They want us to pay more for the G-SYNC enabled models most probably because of licensing.
You can't disable the iGPU in these machines. In fact it's quite rare that you can. The option is usually included in the BIOS but you'd have to hack it open and flash the BIOS with your custom version to make the menu item show up. It's usually not allowed to do this for a reason (hardware-based restrictions) and most of the time (as in my old N61Ja) it won't work either way.
Personally, I would have avoided Optimus-enabled laptops as a whole.
If you're still sure you wanna keep your G751 after these, we can help you out with any other questions you may have...pretty much everything has been discussed on these forums.
Also the touchscreen models are hardly exclusive, much less to BestBuy...