These are points needed to buy certain guild related items. After speaking to a Guildworker's Union Representative and signing up with a Guild, you can trade requested items of your craft to the Guild Representative for Guild Points. The amount of points you can earn each day varies from day to day. GP is not lost when swapping guilds, it simply goes to "inactive" and you can retain it once swapping back again.
See Guildworker's Union for more information.