Please note that you must have a proper ImageJ plugin installed for opening .GEL files!
The pixel values of .GEL files are stored in a way that is more accurate for small pixel values (but less accurate for large pixel values). It does this by taking the square-root of the actual values and applying a scaling factor to make the final values fit in the 16-bit integer range of TIF files.
The .GEL file can be read just fine by any program that can open TIF files. But if you just naively read the .GEL file as a TIF file, then the values will be wrong, because of the square-root transformation (which means that small pixel values will appear larger than they actually are, and large pixel values will appear smaller than they actually are - effectively making the image more "grayed out").
For ImageJ, you need to have a plugin installed that will perform the inverse transformation to get the pixel values back to the correct pixel values. I recommend the BioFormats plugin for this. If you are using the all-batteries-included "FIJI" distribution of ImageJ, then you should already have BioFormats installed, and everything will be alright.