Install the Lilu.kext & NvidiaGraphicsFixup.kext in /Volumes/EFI/EFI/CLOVER/kexts/Other. It says it's for MacPro6,1 but omrij confirms it works for iMac15 and pastrychef confirms it works for the iMac 17 board IDs as well.
My last easy solution is a AppleScript app (AGDPfix.app) that patches the OEM kext's info.plist via shell scripts. OS X updates usually cause the kext to be overwritten/updated so the app must be run again after updating OS X, but it's a lot easier than manually patching the info.plist every time. I originally created v1.0 for MacPro6,1 only but have now updated it to v1.1 for use with iMac15/17 sys defs as well. Feel free to download and use the app attached below. NOTE: You will need to use the boot argument kext-dev-mode=1 for 10.10 Yosemite, disable SIP or use kext-dev-mode in 10.11 El Capitan and 10.12 Sierra.
My preferred method. For those of you using MacPro6,1 and willing to get a little more geeky, assigning the name GFX1 to your GPU in your ACPI tables (DSDT or SSDT) will also work without the need to patch any kexts and you won't need to worry about re-patching anything after OS X updates unless Apple changes the device policy again (credit : dgsga and furter).
#manual
Table of Contents:
Problem 1: Computer not supported error during install
Problem 2: This version OS X is not supported during install
Problem 3: Install the web drivers without the NVIDIA GPU installed
Problem 4: Black screen when using iMac15,1 or iMac17,1 or MacPro 6,1 system definition
Problem 5: Updated OS X and new drivers aren't available yet
Problem 6: No native NVRAM support to remember driver selection
https://www.tonymacx86.com/threads/solving-nvidia-driver-install-loading-problems.161256/