G-Letters is Gokudera's self-invented alphabet. He came up with it during the class hours of first year in middle school due to his boredom. The script's signs include bones, dynamites, skulls, dinosaurs, fish, eyes, and a UFO. Most of them are things that have to do with Gokudera's experiences and interests. Each symbol represents a hiragana character. When Tsuna first saw it, he said that it looked like drawings to him.
The first time G-Letters were seen was during the Future Arc, when Gokudera found a message written in G-Letters among his future self's belongings. It was a message to gather all of the 10th generation Vongola Guardians.