Gokudera grew up in a castle in Italy with his father who apparently is a rich mafioso, and his step-mom. He also has a half-sister called Bianchi; he still gets sick every time she's near him due to her always feeding him poisoned food when they were young (her special talent is poison cooking). She can avoid making him feel nauseated if she covers part of her face.
Shamal, a renowned hitman, used to work for the family as a doctor at some point. Gokudera admired the man so he approached him, wanting to learn his Trident Mosquito technique. Shamal taught him how to attack with dynamites instead, but quit instructing the kid when he came back from a fight, bragging about all the injuries he'd gotten. Gokudera didn't understand the reason for this until many years later.
Hayato took piano lessons from a very talented, kind woman. He was very fond of her, and was pretty sad when she suddenly stopped showing up when he was 3 years old. Five years later he heard the maids gossiping, and realized that the piano-teacher was actually his real mom, and that his father had assassinated her! (In later chapters Bianchi tells Gokudera that his mother was very sick, and that was the real reason why her car slipped of the cliff on an even road. He's unsure if that's the real truth however).
So at the age of 8, Gokudera ran away from home and made up his mind to join a Mafia family to get revenge on his father. Things didn't go as planned however; no family wanted to accept the young pianist, pointing out that he was a weak and troublesome halfbreed(3/4 Italian due to his father, and 1/4 Japanese due to his mother). So he got by on his own, taking a few odd jobs here and there to get by and make a name for himself. He was given the nickname Smoking Bomb due to his fighting technique and his ability to conceal dynamite all over his body.
At the age of 14, a legendary hitman called Reborn gave him a call, suggesting that he should head to Japan and challenge the candidate to becoming the 10th boss of Vongola. Hayato heeded the arcobaleno's advice, and quickly decided to kill Tsuna because he thought that the boy was pathetic and unworthy of the title. During the struggle, Gokudera lost control of his dynamites so they were about to blow him up instead of his target, but Tsuna demonstrated what he was really capable of and saved his life. Gokudera was so touched and amazed that he vowed to follow the fellow teen from that day forward.
Gokudera doesn't approve of most of Tsuna's growing circle of friends at first, but he accepts them after a while. He becomes expecially jealous when Tsuna befriends the talented and popular baseballplayer called Yamamoto, but later on learns to accept him too, and even trust him.
The Family's first real fights are with Mukuro and his gang. Gokudera fights against Chikusa and his deadly yoyo, and think he has won when Tsuna shows up. Chikusa reapears, attacking the soon to be-mafia boss, but Gokudera takes the hit instead. Yamamoto arrives in the brink of time and chases the attacker away, and Gokudera is sent to the hospital where Shamal patches him up. He later later get's to fight against the yoyo-guy again, but his injuries act up, and Chikusa gets help from Ken. They are about to attack him, but he manages to free an imprissoned Hibari that helps him out. Gokudera is also one of the people possesed by Mukuro later on, and has to fight Tsuna against his will. Tsuna manages to save everyone in the end though, and Mukuro and his friends are imprisoned.
The next big fights are against the Varia. Squalo arrives, and beats up Gokudera, Yamamoto and Tsuna. The friends figure out that they need to train in order to get stronger, and Hayato asks his former mentor to help him out. Dr. Shamal refuses to teach him anything at first, but later gives in. He gives Gokudera tips on improving his techniques, and helps him create the rocket bomb.
He use these tricks in the fight against a enemy Storm Guardian, Belphegor. It's difficult to figure out Bel's attacks at first, but Gokudera manages to beat him in the end. But Bel uses his remaining strenght to hinder him as Hayato is about to pick up the second part of the Storm Ring which he needs in order to confirm that he won the fight, and they continue to struggle. Tsuna and Shamal shouts that Gokudera should give up and return to them since the timelimit of the battle is about to run out, and a bomb is about to blow the two combatants to pieces - it's not worth dying for! Hayato is swayed, and returns to everyone's relief, but he looses the battle since he left the ring behind with Bel. Later in the Varia fights he battles Levi, using the new tecnique learned by Shamal to defeat him by blowing up all Levi's lightning umbrellas at once. The Vongola family wins in the end.
The next turn of events brings the gang 10 years into the future! This happends when everyone is shot by Lambo's bazoka, a device that makes you switch place with your 10-year older counterpart. Tsuna and Gokudera are the first to arrive, and get to hear about the grim future they hope to change. They also learn that people in this era fight with something called flames and box weapons. A woman called Lal Mirch agrees to teach the Vongola kids how to use this power.
When Kyoko, one of Tsunas friends, vanish, Yamamoto and Gokudera ventures out of the Family stronghold to find her. They get into a fight where the half-Italian tells Yamamoto how much he hates him, but the friendly guy tries to laugh it of as usual. They then meet up with Gamma, the captain of the Blackspell, and Yamamoto suggest that they need to team up in order to take him down. Gokudera refuses coldly, declaring that he'll handle this himself, for once making the cheery teenager beside him angry.
Yamamoto watches as Gamma and Gokudera fight, but finally can't manage to be passive anymore, knocking the silver-haired teenager out of harms way as he gives him a lecture on how selfish he is, and that he needs to open up to more people than just Tsuna. The baseballplayer then starts fighting Gamma instead, but Gokudera joins him seconds later after recovering from the shock of Yamamoto's lecture, saying that it's ok for them to work together after all. The two are friends again, but still can't manage to take down the Blackspell leader. Hibari saves them in the end.
After this they all train even harder. Bianchi offers to train Hayato, but is upset when he runs away. Later on he emerges, along with a fierce little wildcat called Uri, revealing that he learned the new tecnique C. A. I. by himself. He uses this way of fighting when he meets Gamma again. The two are on par now, and end up with a draw were both are too hurt to fight any longer.
Many fights and turn of events later, the group manages to defeat the main enemy, and are able to return to the past.