Are you the last character in the guild and thus the guild-master? If so you generally can't simply leave the guild but have to permanently disband it with the command /gdisband instead of the usual /gquit. If there are other characters left, you could try to put them to the guild-master rank and as a normal member should be able to leave without any hindrance by the guild bank.
If you are alone in there but don't want to destroy the guild another option would be to call out on fleet chat for someone who wants to take the guild over from you, invite them, give them the rank of the guild-master and you should be able to leave by the /gquit command.
