What is Gastroenterologist??

My 10 week old has been gaining weight very slowly (been monitoring her weight) and now her pediatrician is very concerned and is referring her to a GI specialist. I looked it up and it stands for Gastroenterologist.She was Born May 14 5.6 pounds and today she weighs 8.2 pounds @ her 2 month appointment. She has had problems with reflex and spit up so I hope it's nothing bad. Right now shes taking Ranitadine and gentlease formula.We are waiting to get a call to set up the appointment with the specialist. Just wanted to know if any moms had to go to an appointment like this and what to expect during the appointment? She's on the left and her cousin is 3 months on the right :/