My personal (Watsonian) theory on how Gale's abs are visible
Ladies, gentlemen, nonbinary dazzlers; you know her, you don't love her, you do everything in your power to keep Gale from detonating her...
Now, hear me out. It's canon that this thing eats Gale's magic (a.k.a. the Weave). It's also canon that the Orb is or could be affected by Gale's emotions and/or physical body (a.k.a. Gale being afraid that arousal could cause it to detonate). Plus, whenever Gale dies in-game his corpse just happens to leak toxic magic which gives everyone in the area continuous necrotic damage.
Thus, it is reasonable to conclude that hosting a Netherese Orb does a lot of physiological damage to the body; damage on par with that of a tumor or a parasite.
Supporting this is Mystra's dialogue with Gale. Namely, she states that the Orb can "never be sated", that it would "consume all magic in existence". That it was a "miracle" it consumed "no more than [Gale's] powers". That she "allows" it to eat real magic, and that this is a temporary solution.
All of which heavily implies, if not outright tells us, that Gale could have been killed on first contact with the Orb (along with the rest of the universe possibly being nuked out of existence).
Therefore, if the Orb is indeed as toxic/world-destroying as canon tells us, it makes sense that Gale's body would require vast amounts of caloric energy in order to keep it contained. In order to acquire/maintain which, Gale's body might resort to depleting its fat stores.
This in turn might create the appearance of abs-especially if Gale is dehydrated. And since, as we have established, the Orb is super toxic and might be a serious drain on his health, he might be.
TLDR: Gale has abs because the Orb takes a serious toll on his body, causing him to lose weight without trying.
