Take a look at this post I made last about how the QTR Linearize function and Gray Curve inputs can differ:
The correction curve I make with the spreadsheet tool is basically a series of lookups and linear interpolations, which I assume is about all the QTR Linearization is doing. The difference is how the QTR linearization generates the splines for the intermediate quad values between each of the control points. The Gray Curve seems to be doing something in a different order and can be smoother in some instances. I've included examples of cases where it will create a correction curve for something that fails to linearize with the QTR curve creation linearization (using the same measurement data file). Take a look at the screenshots in the results at the end of the post to see what i mean.
This isn't to say that one is better than the other, but that this can be a useful tool that can get you closer than more generalized gray curve settings.