GIMP is called the GNU Image Manipulation Program, but that doesn't mean it should do every imaginable kind of image manipulation. Scientific image processing and the things that GIMP and Photoshop are useful for are different enough that to add all of ImageJ features to GIMP or vice versa would make them more confusing, waste developer time and focus, and overall hurt users. And I say this as a user of both.

Just use ImageJ!