Original message from September 24: Players from Belgium cannot currently purchase gems for Guild Wars 2. Both direct purchases and redeeming gem codes are not possible. It is likely that this country-specific change is related to the ban on loot boxes.
Update from September 26: ArenaNet has since commented on the changes. In the official forum, it states that due to changes in Belgian legislation, restrictions have been implemented.
These relate to both the direct purchase of gems as well as the Ultimate Editions of the game. The latter also includes real currency as an additional reward.
The developers are also reportedly looking for more flexible solutions. Since the laws pertain to gambling, it can be assumed that ArenaNet currently lacks the technology to restrict only certain items in specific regions.
Original message from September 24:
The purchase of gems is no longer possible: As a player reported on Reddit, Belgians have been unable to purchase gems since the patch on September 18. The blue button that normally allows in-game access to purchasing gems generates an error message.
Other methods such as redeeming codes acquired from other vendors are currently disabled. When attempting to redeem a code, the corresponding button remains grey.
Additionally, the country Belgium no longer appears when trying to make a purchase from another country during address entry.

Is the loot box ban to blame? In April 2018, the Belgian Gaming Commission announced that loot boxes fall under the gambling category based on specific criteria.
In response, game developers took the following actions:
- For instance, Valve removed all loot boxes from the game CS:GO in July of this year, but only in the Netherlands and Belgium.
- Numerous other companies have implemented special rules for their loot box games in Belgium.
Loot boxes in Guild Wars 2: Guild Wars 2 also features such loot boxes. The Black Lion Chests require keys to unlock. Players can purchase these keys in the shop.
These chests mainly contain items that can also be purchased from other players in the Trading Post for gold. In January, the loot box was made slightly fairer.
However, there are sporadic rare skins that are exclusively available in this chest. Since keys can also be acquired through the exchange of gold for gems, ArenaNet may have decided to restrict the purchase of the currency for real money first before removing the chests themselves in Belgium.

Will players still get gems? Reddit user TheSidFlare has worked on a workaround to purchase gems. With the help of a virtual private network (VPN) and an address from abroad, gems can still be purchased.
The ban currently affects only players with a Belgian IP.
Loot box bans in Germany as well? More about this: