Output not linearised after gamma correction with multiple monitors

Hi, I found this thread on the Google Forum:

I’m having the same issue as described there. I have entered the luminance values measured using a photometer into the monitor centre. However, the output did not give gamma values of 1.

I am trying to calibrate a ViewPixx3D monitor, which is connected to an iMac (Mac OS 10.13.4, NVIDIA GeForce GT 640M 512 MB graphics). I was wondering if the output is not linearising because of the video card and second monitor screen, as Jon suggested on the Google thread.

Any help would be appreciated! Thanks