One of the many things I love about this game is that MOST of our companions are disabled in one way, shape, or form. This is appropriate for a story so much about bodily autonomy and its limits, a story that begins with a quest for a cure. Our companions' disabilities vary dramatically from more metaphorical to more literal, more chronic to more acute, disabilities of the body vs. mind vs. a bit of both. Likewise, each companion varies on whether the happy ending for them is cure for the disability or care for the disability and whether that disability is treated as inherently traumatic or as a neutral fact about the body and mind. The result is a game with a very nuanced and thorough exploration of disability, one that I think deserves more love. Some companion disability breakdowns under the read-more, because apparently I am obsessed with this.
Shadowheart: amnesiac in the chronic pain gang. Her amnesia and chronic pain are both caused by Shar to help control her as a cult member. Her disabilities tie directly into the theme of the loss of bodily autonomy to a bodily defect (the tadpole vs. Shar's wound). However, Shadowheart's happy ending doesn't inherently involve being cured. If she chooses a Selunite path and frees her parents (arguably the happiest ending for god's favorite princess), Shadowheart remains an amnesiac (lacking all memory of her childhood and early adulthood) with chronic pain. It is just that Shar's control over Shadowheart's body/mind is no longer control over Shadowheart. Her arc is a really cool twist on the whole "God gave you disability as a curse/test" nonsense--in this case, that is literally true, but she still lives a happy and fulfilled life with her disabilities and doesn't require a cure.
Gale: as pointed out by OP, Gale has a chronic illness that gives him chronic pain. Unlike Shadowheart, his illness is terminal: he must find a cure or he dies. Also unlike Shadowheart, he is in the midst of a pretty clear depression episode and grappling with suicidal ideation. His happiest ending is finding a cure and living as a professor/retired adventurer. While Shadowheart's happiest ending entails living to the fullest with your disabilities, Gale's happiest ending is about finding a way to live when everything in your body, mind, and environment insist you must die.
Karlach: Karlach, like Gale, has a chronic and terminal condition. In her case, she's a fantasy disabled veteran who's going to die because the fantasy military experimented on her. Her relationship to disability and death is directly foiled to Gale's--but her arc comes with the knowledge that a cure is almost certainly impossible. However, care is not--she can go to Dammon, she can go through fantasy surgeries to manage her quite literal and quite firey flare-ups, and she can find a care network who will emotionally and physically support her as her body fails. When a cure is impossible, it is her care networks that keep her alive--Wyll (and whoever decides to join them) going to Avernus. Karlach's relationship to disability is all about finding joy even in impending death, and learning that care is more powerful than cure. (This is one of the reasons why I personally am happy that Karlach DOESN'T get cured at the end of the game--it would completely ruin the message around the power of care ethics and the futility of cure ethics, a central theme in the whole story!)
Astarion: a vampire. No, literally, that's his disability: the man's a vampire. He cannot walk in the sun, cross running water, enter a home without invitation, see his reflection, etc. He's also constantly deprived of proper nutrients through Cazador's abuse and his condition. Like Shadowheart, he's an amnesiac. Like Shadowheart, I would argue his happiest ending is the one where he isn't cured. The Ascension ritual creates a new kind of vampire: a vampire who has all the perks of vampirism (what academics called "hyperabilities") but none of the disabilities that go with vampirism. But if he refuses the Ascension, he refuses the closest thing he has to the cure--and it is only in this ending that he truly undoes the traumas of vampirism. If he remains a spawn, he stays in a vampire's disabled body, but he lets go of the traumatic and abusive culture of the vampire master/spawn--allowing others to be his equal, allowing himself to truly care for others and to be cared for in turn. ("You give me something to care for, and that is worth the peril.") Astarion in many ways is Shadowheart's foil when it comes to disability--he too got his through lifelong abuse, and he too finds his most fulfilled and healed self in the ending where he is not cured. Like Shadowheart's and Karlach's stories, Astarion's "radiant hopeful" ending is all about the joys that can be found outside a cure for disability. His Ascendent ending, on the flipside, is about the tremendous violence that a cure ethic entails: it "cures" some of the disabled, and relegates the uncurable disabled to die because they are not truly "alive" or "people" (the other spawn who don't ascend, but must be sacrificed for the ritual because it is "kinder" than letting them live). Astarion's endings more openly link cure ethics to eugenics and care ethics to the healing and joy within disabled life.
Wyll: Wyll is interesting in that, aside from the tadpole, his plot doesn't revolve around care/cure for a disability. He only has one canonical disability at the start of the story: his eye, lost in battle, replaced by a prosthetic that Mizora uses to spy on him. Like many of the other companions, his prosthetic eye directly relates to his loss of control over his own body and soul. However, unlike the other companions, the missing eye is mostly treated as a neutral fact of his body, not an angst-ridden and plot-defining aspect of his character. Later, Mizora disfigures him as punishment for sparing Karlach. As he grapples with his scars/horns, he grapples with the narrative that deformity and disfigurement are signs of monstrosity--an old belief that disability revealed the evilness of the soul (think Shakespeare's Richard III). Ultimately, he sees that his deeds, not his disfigurement, define him--and he comes to see his horns and scarring as a neutral or even handsome part of his appearance ("I wish I had time to polish up the horns" in the epilogue). Wyll is about disability as a neutral fact of life, rather than as some trial to overcome.
Lae'zel: our only abled companion, and she's a supersoldier from outer space. You thought Baldur's Gate 3 would give us a "normal" body? You fool--normal is a myth, and Lae'zel sure as hell ain't it. She's an abled githyanki, which means she is parthonogenic, teleports at will, and can see in pitch-black shadow. She is hyperable by the standards of the "weaklings" of Faerun, but abled by the standards of her own people. Lae'zel is a reminder that "abled" is not the same as normal, and that both ability and normalcy are social constructs. After all, to a githyanki, "your large fleshy nose looks like a mistake." While the other companions are about disability, Lae'zel's story is a commentary on ability. Lae'zel's story is about an extremely abled person suddenly facing the threat of disability (getting tadpoled), and immediately floundering in fear of it. She turns to the hope of a cure (the zaithisk). Like so many of our companions do in their personal arcs, she finds that the "cure" is horrific violence done to disabled people--one that will ultimately kill them in the name of scientific advancement.
This...spiraled beyond what I intended. Truly, there are whole essays to be written about disability in this game.
tl, dr; Baldur's Gate 3 is a game about disability and ability, the ethics of care and cure, and the limits of bodily autonomy. Each companion has something to say about these themes, and I think part of loving them is loving what they have to say about disability.