As asus builds custom cards for their laptops, the best chance you have is to find a GPU from a different computer in the same series (e.g. another g750) However it looks like the g750 series is now discontinued in favor of the new G751 laptops and I am not sure if the GPUs in those can be used in a JS. Even if they were compatible, finding a usable 970m would be difficult and expensive.
Asus takes a different approach to gaming laptop design than alienware, sager, etc. They prebuild computers and sell them as a unit. Other manufacturers custom build units for each user allowing for easier upgrades. Even among those, upgrading major mobile components such as the CPU and GPU is uncommon and expensive.
The short answer is most likely that you cannot upgrade.
