A gastroscopy is a test where the endoscopist looks into the upper part of your gut (the upper gastrointestinal tract). The upper gut consists of the oesophagus (gullet), stomach and duodenum. The operator uses an endoscope to look inside your gut. Therefore, the test is sometimes called endoscopy.An endoscope is a thin, flexible, telescope. It is about as thick as a little finger. The endoscope is passed through the mouth, into the oesophagus and down towards the stomach and duodenum.The test is usually carried out with either a local anaesthetic throat spray, or a light sedative.