The GRADIENT_9.DAT is located in the folder <glass> which is set in project preference.
It can be opened by any text editor. Once it's opened, we can add a new material.

The first line in the file is the name of the material, which can be any name (without special characters (javascript:void(0);) such as spaces or quotes) less than 20 characters long. 
The next 5 lines are the values for B, C, K0, K1, and K2. There are no blank lines allowed between blocks.
Here is how these parameters are used:

Once the parameters are edited, we can save the GRADIENT_9.DAT file and set it in lens data editor.