A GI specialist (in this case a pediatric GI specialist) is a gastroenterologist, and you should be seeing one who works just with children and adolescents.
Gastroenterologists deal with all aspects of the digestive system. This includes the mouth, swallowing, the esophagus (the tube going from the mouth to the stomach), the stomach, the small intestines, the large intestines, and the rectum.
GI doctors also deal with the organs that work with digestion such as the liver, gallbladder, and sometimes the pancreas (although the pancreas is more complex and problems with the pancreas are often dealt with by an endocrinologist.)
The GI doctor is going to start by taking a very thorough history. He will want to know all about the conception and pregnancy, the delivery, development, and everything that has happened in the past (almost) 2 years.
Then the doctor will do a physical examination, much like the pediatrician. He will check the baby's weight, length, listen to the heart, lung, and stomach, feel the belly, etc.
There are procedures that the doctor can do, but this depends on what the symptoms are that is causing you to bring your 21-month old to see him.
At this point, can you tell me more about the problems that are causing you see to bring your baby to the GI doctor?