While fans of the Bruce Banner's gamma exposure turned him into what Marvel branded "the strangest man of all time!!" It's Hulk's origin that shows how true this boast was.
While Marvel has explored and expanded on the lore surrounding gamma mutation - recently linking it to the cosmic being known as the One-Below-All - Hulk's original adventures came up with the rules of his transformation on the fly. Bruce Banner would transform into the Hulk at night and regain human form during the day, later triggering his changes via a gamma ray before finally settling into the modern status quo, where strong emotions prompt him to "Hulk out." However, in these early days, it wasn't just the method of transformation that was unpredictable.
While Hulk has always been strong and tough, his powers were originally far less consistent. Hulk's body would respond to threats in new ways, with his monstrous form and warped personality offering up countless surprises. In the first six issues from Stan Lee, Jack Kirby, and Steve Ditko, Hulk is rendered super-suggestible, switches personalities and colors, and even transforms everything but his head. The idea of Hulk as a brute who gets stronger with rage wasn't yet established, and the stories focus on the disturbing idea of awakening a monstrous and truly unpredictable inner self. These body-horror powers remain a part of Marvel continuity, and are best explained by Bruce Banner in New Avengers: Ultron Forever #1 (from Al Ewing, Alan Davis, Mark Farmer, Rachelle Rosenberg, and Travis Lanham.) After Hulk gets his head off and Banner's grows out of his chest to replace it, Bruce explains to his time-traveling allies:
The curse of the Hulk... warps my body. It's different every day, every month. Like the monster's searching for what to be. ... Perhaps in the future, the Hulk is something predictable, but right now... I'm afraid this is not an abnormality.
These foundational stories have been essential to making the Hulk more than a super-strong brawler, adding a deep metaphysical context to the forms Hulk takes and what he can and can't do. While movie adaptations tend to depict the classic 'Savage' Hulk, this isn't the original version of the hero, just the most simple iteration. There have been very few eras in Hulk's publication history where creators ignored the infinite potential of Hulk's transformations, though a modern highlight is Al Ewing and Joe Bennett's Immortal Hulk, which returned fully to the roots of Hulk's body as an unknowable source of horror. It's the Hulk's potential to do and be anything that makes him so terrifying to Bruce Banner, providing essential context for their acrimonious relationship even when the Hulk is at his most stable and heroic.
Hulk was a pop culture artifact of very real fears about the potential of radiation to change the human form, and the unstable period directly following his origin is the purest expression of this idea. While Hulk is the Avengers' big, angry muscle, he's also the warped expression of Bruce Banner's repressed trauma and rage, and the gross, disturbing things his body can do are essential to that aspect of the character.
