One of the most powerful warriors in all of Dragon Ball is Goku and Chi-Chi's son Gohan, whose mixed Saiyan and human heritage made him a formidable fighter from a very early age at the start of Dragon Ball Z. While Gohan's father trained him on multiple occasions throughout the manga/anime franchise, the young Super Saiyan opted to wear fighting gear modeled after Piccolo instead of Goku for much of DBZ.
In choosing this distinctive attire, Gohan has demonstrated how much Piccolo means to him, not only as a mentor but as a surrogate father figure during Goku's lengthy absences in his life.
At the start of DBZ, Piccolo was still very much Goku's most significant enemy after previously trying to follow in his father's footsteps to conquer the world. The two rivals agreed to set aside their differences to work together and defeat Raditz -- Goku's evil long-lost brother who attempted to wipe out humanity. The resulting battle ended with Goku dead and Piccolo deciding to train Gohan in anticipation of the arrival of even more powerful Saiyans that would arrive on Earth in approximately one year.
Piccolo trained Gohan to tap into his latent potential and become a strong warrior in his own right. Complete with a version of Piccolo's Namekian clothing, Gohan's influence made Piccolo appreciate humanity and begrudgingly turn from wicked ways to become a committed Z Fighter. The bond between the two became apparent when Piccolo heroically sacrificed his life to protect Gohan from Nappa, leaving the boy visibly heartbroken.
For much of his time on Namek, Gohan retained Piccolo's clothing, only switching to Saiyan armor to give him more protection ahead of the impending fight against Frieza. After training with Goku and Piccolo for the prophesied Androids' arrival back on Earth, Gohan kept his usual Namekian clothing in battle. However, he took the outfit one step further after training with Goku in the Hyperbolic Time Chamber for a full year in preparation for Cell's tournament.
In addition to the purple and white garments, Gohan began wearing Piccolo's weighted cape to make the association more apparent, with the wardrobe change as a way to honor his original martial arts mentor prominently. Gohan would wear this outfit for the entirety of the Cell Games when he unveiled the Super Saiyan 2 transformation and saved the world.
While Gohan would develop his own green and black costume to masquerade as the superhero the Great Saiyaman, he decided to honor his father after having his full potential unlocked by the Elder Kai and achieving the Mystic transformation. Recognizing Goku as the greatest fighter who sacrificed so much -- including his life on multiple occasions -- to defend the universe, Gohan finally began wearing his father's iconic orange and blue outfit. His version lacked Goku's Turtle School symbol on the gi as he had not been instructed in Master Roshi's fighting style.
Gohan has had more wardrobe changes for fighting outfits than the rest of the Z Fighters throughout DBZ. He started the anime series wearing a variation of his Grandpa Gohan's outfit before taking on many different styles, usually inspired by his original mentor Piccolo. While he would take more after his father, fashion-wise, as he reached adulthood, Piccolo has continued to remain a significant figure in his life as Gohan became a father and resumed his training throughout Dragon Ball Super.