If you are going to use the PC for gaming only there is no reason why you would need GTX 1080, just a waste of money imo. If you want to get the maximum performance from your games you could get 1060 although most games won't even need that much.
Also as far as I've read Asus VG248QE has horrible viewing angles and its main feature is USB ports which you probably won't even use. I'd suggest using the money you save from downgrading a graphics card and get something like ASUS PB287Q