Co-GM is a Guild Rank just below Guild Master.This Guild position was not part of the original Pirates Online game, though many guilds (especially larger ones) would give members this rank in name only as part of their command structure.
In Legends of Pirates Online, this rank functions very much like Guild Officer, with the additional abilities of being able to promote or demote other member ranks like Officer or Veteran, as well as be able to remove any member from the guild.
This rank can only be assigned by the current Guild Master and there is currently no limit how many members can be named a Co-Guildmaster.
In the case of the Guild Master's removal from the guild or their pirate being deleted, a Co-GM will be promoted to become the new Guild Master in their stead.