Several players from Belgium started reporting issues with Guild Wars 2 in-game store since 19 September 2018 where they couldn't buy the in-game Gems currency. ArenaNet haven't officially said anything yet, but it's probably loot boxes.
Guild Wars 2's in-game currency, Gems, is not directly gambling so at first it might seem odd that Belgium prohibited their purchase but the problem goes a bit deeper. The same currency is used to purchase Black Lion Keys, which are used to open Black Lion Chests, which are ArenaNet's version of loot boxes, so the reason for blocking these transactions in Belgium is pretty obvious.
The gambling doesn't stop there as players can use Gems to purchase Dye Packs and other boxes that can yield random items, which could also be classified as gambling. Therefore, purchasing Gems can sometimes be just gambling with extra steps, rather similar to how one would purchase casino chips and then go gamble with them.
That sentence specifically states "sometimes" because the currency can be used to purchase cosmetics, gold and quality of life upgrades without having to gamble for them. The problem arose when several Belgian players inability to purchase gems at all, even though the currency has many other uses.
While we always consider caution with paid loot boxes a good thing, Belgian players are also denied all other Gem-fueled services for the time being. That said, it's highly unlikely that ArenaNet hatched some evil plan to discredit the Belgian legal system, but they will have to rework how Guild Wars 2's in-game store works.
Needless to say, that kind of work is not a matter of minutes and could take a while until Belgian players are able to purchase Gems again, but they will likely never be able to use them to purchase Black Lion Keys or anything related to unidentified dyes.
There is currently no word from ArenaNet on the matter, but considering that the issue has quite the following now, they will soon have to respond officially.