Yes that is correct, these late 2013 iMacs (I've three of them) do have NVIDIA GeForce GT755M 1024Mo graphical processor unit which seems to run cooler.
I do have mid-2010 and mid-2011 iMac (total of two) with respectively ATI Radeon HD5670 512Mo and AMD Radeon HD6970M 1024Mo graphical processor units, the last failed two times due to the BGA ball game.
Now to go back to the Late 2013, see pictures again after say waiting 2 mins booting (phase 1, phase 2 and phase 3). I still do not understand why PartialDischarge and kripton2035 conclude the GPU fail (the GPU is on main logic board, not on display unit) because all green LED's are lit. Furthermore from my memory, when GPU fails on my older 2011 iMac then very quickly at boot no display or just plain display with question apple mark. Unless I'm wrong, the iMac seems to follow the different booting steps with final step of waiting for my username password.
I attach another picture, we see both ribbon cable (one for video data I guess and the other for display power supply I guess), could one ribbon or its respective connector be failed ?
Has any member on the forum deconstructed these LCD display to then check their internal electronics, are there a local inverter or power supply inside... any guidance much appreciated.
Albert