Guild Marks are a special currency used by the various guilds all around the land of Eorzea in Final Fantasy XIV, each one with its own marks. Therefore, a Gladiator guild mark is useless at the Thaumaturge guild.
Guild marks are obtained mainly via Guildleves of rank 20 and above, replacing the usual gil reward. For disciples of war, magic and land, it instantly recognizable if a guildleve will give marks or not, as it will be shown at the quest description. As for disciples of the hand, only when delivering the requested items that they may receive marks - with a higher rating increasing the chance to receive marks.
The second method to get guild marks is by completing the Class Quests, starting at rank 20. Upon completion, the player is rewarded with substantial gil and guild mark rewards.
There is no way to sell or trade marks with other players or NPCs.
Guild marks are the only currency accepted at the guilds. They are used to buy special abilities for each class, with new abilities becoming available after each Class Quest finished. In the Disciple of the Land guilds, players also have the option to buy shards and crystals using guild marks.