Short answer: yes.
Long answer: It hasn't been exclusively due to GERD, however over the past 10 years, my reflux has accounted for the majority of my weight loss (80lb of it). My reflux started, hard, when I was about 290lbs. I'm now at 210.
Recently (in the last few months), reflux has directly resulted in my losing 10 lbs. All in all, my major drops -- from 290 to 260, 260 to 220, and later 220 to 210 -- have been directly due to reflux not allowing me to eat, or fear of reflux, which leads me not to eat.
And yeah, right flank pain seems to indicate gallbladder. Fun. Been there, done that. After 4 years of it, decided to get it out. I don't miss it, but I don't fool myself into thinking it was entirely without consequence.
All I can advise is eat small, frequent meals, make sure they're high-nutrition, and make sure they're alkaline where possible (so basically stock up on veggies, keep it relatively low fat, and consider a GERD diet). Listen CLOSELY to your body: track symptoms with journal. Also, food. Also, your emotional state, because that matters more than...well, more than I ever thought.
Your weight loss will eventually slow down. It may still keep going, but work to make sure you're not hitting any nutritional deficits. Since you say you're overweight, use this to your advantage. A good multivitamin can help here. (Expect to pay. But yes, a good multivitamin is worth it.)
For me, drinks like Huel and Soylent have worked OK, but I do get some refluxing from them, so I supplement them with something like a piece of bread. Protein shakes are good, too, because they deliver a lot of nutrition (but not a lot of calories; I'm personally a fan of the Vegansmart line). The goal is to make sure you're getting in no less than about 1500 calories per day. (Don't do it in just protein shakes like Vegansmart.) Yes, you'll lose weight, but this should be enough to keep your energy levels up and for you to make sure you keep your nutritional requirements in check.
Best of luck!