Which Gray Gamma to use for Digital Negatives

http://youtu.be/prNBUxlG4Ms

Gray Gamma 1.8 or Gray Gamma 2.2, which one do you use?

Here is a link to a PDF supplement to this video.
GrayGammaSettings.pdf

and, a link to download a 21 step wedge that is 10.5 x 4 inches
made in Adobe RGB (1998) / Gray Gamma 2.2
21StepWedge.tif