The Guild Wars mechanic for Ashes of Creation is still very much in active design. While we know a few things about them, we do not have a comprehensive or exhaustive list of mechanics for the guild war system. We know Intrepid Studios intends to make these meaningful fights with actual objectives, not just hour-long fights to see who accumulate the most kills in the combat window. These objectives are going to be akin to the castle sieges just on a much smaller scale. They are going to give the Guild Wars meaningful gains and impactful consequences.
Prime Time Objectives
As there will be meaningful objectives for the Guild Wars, those objectives will only occur during the Prime-Time Window for the server.
Some of the objectives will revolve around Guild Halls and Guild Fortresses. It may involve infiltration of a Guild Hall and stealing a quest item that spawns during that prime-time window for the Guild War. It may involve sieging and destroying a Fortress the enemy guild holds. It may be to kill a specific guild member, who becomes the target; gaining additional health and/or damage mitigation against members of the enemy guild.
As previously discussed, the Guild Fortress idea may not end up making it into the game or it may be delayed until after launch. We are waiting on an update from Intrepid on this.
It is possible for a guild to be involved in multiple wars at the same time both from a perspective of declaring multiple wars, as well as from the perspective of having multiple wars declared on you. Additionally, it is possible to become entangled in the wars of your alliance members wars as well.
We do know there will be a cost to declaring war and the price increases with the total number of wars you are currently engaged in. What the price is, and what the increase curve is, we do not know. That level of information will come out later in Beta or at launch as Intrepid wants to keep many things close hold as to not ruin the surprise.
Guilds will be ranked based on their performance in both Guild Wars and Castle Sieges in the leaderboard rankings. Right now, there is no known tangible benefit to this ranking beyond bragging rights.