The gall bladder is the reservoir where bile produced by the liver is stored. Gallstones and Cholecystitis (inflammation of the gall bladder often caused by gallstones) have often been found to occur in patients with acid reflux. Whether one causes the other or both share a common cause isn't clear with research papers contradicting each other.
A paper published in 1936 in the Annals of Surgery reported a correlation between acute cholecystitis and reflux [b-ii] though a 2001 paper published in the American Journal of gastroenterology found no association between gallstones and gastroesophageal reflux disease [b-iii].
Gallstones are hard deposits which can form in this bladder. Frequently they cause no problem but sometimes may result in considerable pain. It may not be clear what causes gallstones. they are composed of "biliary sludge" that usually contains cholesterol.
It is suggested some "cholesterol reducing" drugs can result in a build up of cholesterol in bile. Other suggestions are the gall bladder isn't emptying efficiently or frequently enough or the liver may excrete an excess of cholesterol to the gall bladder.
Cholesterol is an essential component of cell walls. it is also used by the liver to make bile, it makes Vitamin D and adrenal and sex hormones. It is transported around the body in the bloodstream by lipoproteins which come in two sorts, Low Density Lipoproteins (LDL, when we call it "bad cholesterol") and High Density Lipoproteins (HDL, when we call it "good cholesterol"). It's the LDL which can cause arteries to block up leading to heart attacks.