The function within nip2 used to preform this correction is Tasks / Capture / Flatfield. A set of images can be corrected at the same time by joining them together in a group. A group can be produced by selecting all of the required images and then using the Edit / Group command.
Load all of your images into nip2, and group all the image except the grey image. Select your grey image and then your new group and then run the Tasks / Capture / Flatfield function. This will produce you a group of corrected images. Right-click on this new group and select Save As from the menu. In the save window type in a name, for example fred_01.v and then hit the save button. All of the images in your group will then be saved as fred_01.v, fred_02.v, fred_03.v ... fred_n.v.
If you want to keep row numbers in your file names, (see §3.2.2), you will need to correct your images one row at a time, saving each row as fred01_01.v, fred02_01.v, etc.