Guild marks are a currency that you use to buy various items in your stronghold, like the powerful Dragonflight armor. You get guild marks from donating practically any items in the game to the mimic in your stronghold. Some things are more valuable than others when it comes to guild marks. You can either farm for your guild marks or you can buy vouchers and other items from the auction house to donate. Either way, doing this during a double Guild Mark event is preferable.
Farming Guild Marks
The cheapest way to get Guild Marks is by farming items yourself that you can donate. And the most stable way to farm it is doing Heroic Events in your guilds Stronghold. If you have a big and active guild, most guilds will do runs most days where they do the epic Heroic Events. The advantage of doing the epic ones is that they give additional rewards in addition to the influence that you will use to donate for guild marks. These rewards include resonance stones and companion only gear. However, all Heroic Events give the same amount of Influence and is capped daily at 400. The mobs in the Heroic Events also drop vouchers that can be donated for extra Guild Marks. The smaller Heroic Events can be soloed by most players. I recommend doing these daily for free and easy Guild Marks.