I'm coming into this discussion late...

But the standard main menu items appear to be defined by a series of .xml files contained in the GIMP program directory at "GIMP-2.0\share\gimp\2.0\menus".

For example, the file "GIMP-2.0\share\gimp\2.0\menus\image-menu.xml" appears to define the main window menu structure for the standard GIMP items. It appears that some of the sub-entries that appear on screen are filled in dynamically as GIMP starts, and everything else is appended to the bottom of the menu in alphabetical order.

I don't feel the urge to mess with those files, but the option exists for someone who really wants to customize their menus.