Gandalf the green is a term used for a marijuana dealer that messages you notifying you that he has just got weed if you are looking for it when all other dealers have failed you.
Guy 1: "You get sorted?"
Guy 2: "Nah man, all my guys are sacked, you?"
Guy 1: "What do we do now?"
Guy 2: " We go home, you fool!"
Guy 2: "Oh... Wait"
Txt message: "I am Gandalf the green, and I can get you sorted now... at the turn of the tide."