Ever since the Saiyan Saga, Gohan has been one of the most important characters in Dragon Ball. Despite telling mostly the same story, his exact level of importance differs greatly between the DBZ anime or Dragon Ball manga. As much as Dragon Ball Z's Gohan may mean to the kids who grew up watching him, this version of the character was not true to Akira Toriyama's vision.
Gohan is written similarly in the Dragon Ball manga and the Dragon Ball Z anime, but the way in which he's portrayed differs greatly. In the manga, up until the moment in which he fights Cell, Gohan is just another member of the main cast, no different from Vegeta or Piccolo. The anime, however, is filled with non-canon filler that goes out of its way to present Gohan as a co-protagonist to Goku, which fundamentally changes the role that he plays in the series, and the idea behind what his story is meant to be about.

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From Dragon Ball Z Episode 1, Gohan is focused on as if he is the main character. When not focused on Raditz, the episode mainly plays out from Gohan's perspective as he gets into trouble in the woods. Especially for those watching who had never seen the previous Dragon Ball anime, the series is made to feel like a story about the young, meek son of a great warrior who will one day surpass his father as Earth's hero. As the Saiyan Saga progresses, Gohan's potential power is almost immediately shown to be greater than Goku's, his training in preparation for the Saiyans is given equal screentime to Goku's, he undergoes character development while Goku doesn't, and he's ultimately the one to defeat Vegeta.

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Beyond the first arc in DBZ, Gohan is the main hero for most of the Frieza Saga. The Cell Saga sidelines both Gohan & Goku until it concludes with Gohan surpassing his father like everyone knew he would, while the Buu Saga begins with Gohan as the singular main character. On top of all of this, Gohan is the protagonist of the Garlic Jr. Saga While he's only the main character of a singular DBZ movie, he's frequently given more narrative focus in them than his father. Even before he's able to fight alongside Goku and the rest of the Z-Warriors, the movies compensate by giving him musical numbers and a pet dragon, Icarus.
, and this isn't just conjecture based off what they wound up producing. After it was decided that the Dragon Ball anime would come to an end and Akira Toriyama's Dragon Ball manga would continue to be adapted into a sequel series, one of the first titles they considered was "Dragon Ball: Gohan's Big Adventure." It was clearly felt that, since the original Dragon Ball was focused on Goku's journey from childhood to adulthood, Dragon Ball Z should follow the same successful formula – even if that wasn't true to the manga.
Manga Gohan shares the same personality, relationships, design, and transformations as DBZ Gohan, while also experiencing most of the same struggles. That said, the ways in which their stories are framed couldn't be more different. This is done not only through overt changes, like filler episodes and the existence of the Dragon Ball Z movies, but through more subtle direction choices during critical moments.
While Gohan is important in the manga's version of the Saiyan Saga, his first appearance is when Goku brings him to Kame House to meet his friends. Gohan's misadventures in the woods in Dragon Ball ZEpisode 1 are not in the manga, and the main perspective among the heroes in the manga never stops being Goku's. Likewise, most of the time spent focusing on Gohan's training in preparation for the Saiyan invasion is all filler, as well. The manga version of the Frieza Saga is significantly shorter than the anime's version and, without the filler plots set before the Dragon Team arrives on Namek, Gohan merely reads as part of a core ensemble, rather than a protagonist.

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Between this, the absence of the Garlic Jr. Saga and all the other filler and film storylines, and the way the Cell Saga keeps Gohan in the background for most of its run, Gohan being the one to defeat Cell in the manga winds up being played more as a twist than an inevitability. . Throughout the series, Goku is the one who takes part in all the major fights, Vegeta is the one driving the action, and Gohan is merely just the main representative of the Z-Warriors as a group.
In both the anime and the manga, Gohan is presented as the central hero from the time he ascends to Super Saiyan 2 up through the point Ultimate Gohan loses to Super Buu. While the anime constantly goes out of its way to make Gohan feel more important, the majority of the first three sagas in the manga do nothing to make Gohan feel like Goku's equal. He's a main character in the same way that Vegeta, Piccolo, and Krillin are main characters, but the story remains Goku's as it always had been previously, and nearly always would be in the future.
Up until the release of Dragon Ball Super: Super Hero, it was a common criticism that DBS wasted and disrespected Gohan. For those who have only ever experienced the series in anime form, and especially those who never watched the original Dragon Ball, this makes sense. Without any knowledge of what's canon and what isn't, . In both the anime and the manga, Gohan is ultimately robbed of his position as the Buu Saga's protagonist, and Goku didn't relinquish it back to him when the franchise returned in 2013.
Akira Toriyama was heavily involved in the films which brought Dragon Ball back into the public consciousness, Battle of Gods and Resurrection 'F'. From his perspective, Gohan being the main character was only ever a failed experiment that barely lasted a few chapters, with him wasting no time bringing Goku back into the picture. Most of the material with the Great Saiyaman in the anime is filler and, even if it didn't feel like Gohan was replaced by Goku until the Super Buu fight in the manga, Toriyama knew the moment he brought Goku back to life that Gohan didn't work for him as a main hero. With the two modern Dragon Ball Z movies, it made sense to Toriyama to include Gohan only as a minor supporting character, while focusing on the characters who'd wound up being the biggest heroes and most popular characters by the end of manga's run, Goku and Vegeta.

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By the time Dragon Ball Super started telling original stories, Goku and Vegeta were more heavily focused on as the protagonists than ever, and there was simply no incentive for Toriyama to shift back to Gohan. Even when he finally got cut a lucky break and was made the big hero of Super Hero, that wasn't Toriyama's doing. The most recent Dragon Ball Super movie was made to give Piccolo, Toriyama's favorite character, a power-up, and Gohan's role was only added later, which is why he feels so tacked on.
Kids who only grew up watching Dragon Ball Z may not know Akira Toriyama's Gohan, but that doesn't make the Gohan they do know and love any less valid. The anime and manga telling the same story while focusing on different elements is fascinating, and makes it especially worthwhile to experience the series in both mediums. And the more someone understands the differences between the two, the easier it is to understand how Gohan has wound up where he is now.