UPDATED with a statement from ArenaNet on the inability of players in Belgium to purchase gems. It is NOT a bug.
Update on the Purchase Status for Belgium
In order to conform to changes in Belgian law, purchases of gems, the Ultimate Edition, and the Ultimate Edition upgrade have been disabled for residents of Belgium. We are continuing to investigate more flexible solutions. We apologize for any inconvenience this may cause our players.
Belgian Guild Wars 2 players are seemingly unable to purchase the game's premium currency, Gems. According to forum posts and Reddit, players can no longer enter Belgian addresses as part of billing options. The issue appears to have first started on September 19th when players attempting to buy Gems found an error message even when using a VPN.
It is fairly obvious that this problem for Belgian players is a bug, but it is a curious, and very sudden, occurence as it is an issue only with Belgium, the country that has been cracking down hard on loot boxes and other forms of what its government has determined is "gambling" in online games.
I'd say the reason is underage gambling laws recently introduced in belgium. You can buy gems with real money, convert those gems into gold, then use that gold to ecto gamble until it's all gone/you've become rich. Either that or black lion chests.
Gems are purchased in order to buy items from the Gem Store or to traded to other players via the marketplace for in-game gold.
No statement has been made by the Guild Wars 2 team, though it could happen later today once they are made aware of the situation. In the meantime, head to the links above to learn more.