Apple Cider Vinegar is great for some GERD sufferers, especially if they have issues with low stomach acid production. But GERD is usually a much more complicated issue than just low stomach acid or too much stomach acid production. There's also enzymes that the body produces like Pepsin and Bile. There's also the sphincters that start from your neck that go all the way down your LES (lower esophageal sphincter) and more.
For someone like me who suffers from GERD and esophagitis, apple cider vinegar should NOT be taken. The ACV irritated the heck out of my esophagus and my stomach lining because I also suffer from chronic gastritis. I think in my specific situation ACV only made things worse. However, I do have friends who have GERD only and the ACV with some water helped them a lot. So it depends on what you specifically have.
I have found something good for someone with my condition, I have found more relief from drinking 1-2 tablespoons of sauerkraut juice after each meal. My body tolerates it okay and you can gradually add more to your diet. It gives me the probiotics I need to help break down my food and it also encourages the stomach to produce a little more acid, which also helps break down my food.
Organic sauerkraut is getting expensive, though, so I'm starting to make my own. You can find plenty of recipes online on how to do it. It's simple and once you get your first few batches going, you'll never run out.