This is s specialist of the Gastrointestinal System, including the esophagus, stomach, intestines, and liver and gallbladder (see Diagram). They perform tests like endoscopies, colonoscopies, and various others, make diagnoses, and treat. Those who deal with the liver and gallbladder as a sub-specialty are called hepatologists. None of these specialists do surgery; they refer to surgeons.
Note that the specialist is a gastroenterologist, whereas the system is “gastrointestinal,” often called “G-I”. “Entero-“ means “intestine in Greek, but in medical shorthand, we use the English (“I’ll refer you to G-I;” not ‘to G-E’).