GvG is guild vs guild in which guilds, obviously, fight each other.
The winning guild receives 180 event coupons.
The thing you mean with castles is most probably the War of Emperium.
When a guild takes on a castle, they own it unless another one breaks their emperium.
The guildmaster of the guild that owns the castle has access to the castle's treasure room, where the treasure boxes are. These treasure boxes spawn at about 00:00:00 Server Time.