How do people typically get gerd?

It depends: Being overweight is probably the most common cause but others include a hiatal hernia, dietary indiscretion and bad luck.

Multiple: Gerd is multi-factoral. At its basics, it is a dysfunction of the "valve" between the esophagus and stomach. The function of this valve can be diminished by excess weight, dietary substance, some muscle relaxing substances, etc... Also, individual anatomy and/or changes with age can cause dysfunction. Unfortunately, longstanding gerd can cause further damage and progressive worsening of gerd...