Guildtell is a method by which you can communicate with all of your guildmembers currently online. There are really two kinds of guildtell, with an additional option on one of them. 

Normal Guildtell
The command to use it is GT <whatever you want to say>. GTOFF causes you to stop listening to guildtells, while GTON will cause you to begin listening once again. 

Secret Guildtell
You may also use GTS (for GTSECRET) to speak to everyone in the guild who also has the GTS privilege based on rank or position. The title of the channel will show up with << >> around it to indicate that it is secret. 

Note: GTS is an investible privilege and will only be heard by anyone with it.