There are 2 ways the guild treasury to be filled with gold.
The good: Expect and do donations. You can donate gold & diamonds by going to guild treasury menu.
The bad: Leader and First Officer have the rights to apply tax to the guild members. This way every income the members have from selling items to NPC shops, working at tavern or gold drop from dungeons, will be taxed and the tax will added to guild's treasure.
And always keep an eye for "rats*"...
*->those members with rights over some upgrades (i.e. outpost Commanders) they may be upgrade and spend more than the guild needs or uses.
P.S. If you can't donate any gold or can't add gold with taxes, then there is an issue, so post name of the account, realm and guild's name, and I will move the topic to bugs...