If I already have problems with gas, will a nissen fundoplication necessarily cause worse problems? This is biggest concern.

It could. . Bloating is a common side effect of a nissen fundoplication. With your concerns, you may want to consider a tif or linx. Both are very successful in treating reflux with a much lower incidence of side effects.
Sometimes. The nissen fundoplication procedure is often performed about a certain size bougie inserted through the junction of the food pipe and stomach. At times this is small, making burping impossible. Too large a bougie can result in continued reflux. That is the art of doing the procedure. Ask your surgeon his success rate.