Belgium here... I'll try to keep it short and simple: most stuff requires a gun permit/license, and even more stuff requires an import license.
You need a permit/license for most firearms and ammo, but also for frames, barrels, slides, cylinders, uppers, lowers and bolts. You don't need a license for scopes, optics, sights and magazines.
An import permit is required for all the above, plus everything that goes on or with a weapons, even the smallest spring or part. This includes scopes, optics, sights and even grip plates. In short, ITAR-exempt in this case only means that you can get your stuff out of the USA without any trouble, but that's about it. The receiving party in Belgium will also need an import license and the proper permit/license to own the part in question. Also, bear in mind that certain things will need Belgian proof marks (extra costs and paperwork) or are prohibited altogether (5.7 ammo, for example).
Any questions, PM me for info. I am fairly knowledgeable on the laws (disclaimer: Belgian gun owner and LEO), or can get you in contact with people who know even better.