This article[1], paragraph 2:

    Even as many doctors subscribe to the myth that eating
    fat and cholesterol leads to gallstones, research 
    indicates that eating too little fat and too many 
    carbohydrates in the form of grains, sugars and 
    starches actually leads to gallbladder ailments. ...

Or on WebMD from over a decade ago [2] .. as to the saturated fats... generally animal sources of fats are about half or less saturated, and generally a good mix.

[1] https://www.atkins.com/how-it-works/library/articles/dealing...

[2] https://www.webmd.com/digestive-disorders/news/20041004/more...

“Both extremes are bad” applies to everything, by definition of it being extreme.

Fair, though usually "drinking zero water" as an extreme does not cause death by water intoxication, and avoiding all alcohol does not usually cause liver issues.

So im curious how this particular extreme affects the galbladder.

See effects of too much potassium vs too little, or magnesium, sodium, phosphorous. The symptoms overlap because when the balance is off, it can have similar effects.