A gastroscopy is a procedure that involves inserting an endoscope (i.e., a long, flexible tube with a camera on the end) through the mouth, down the oesophagus, into the stomach, and into the first section of the small bowel (duodenum). Gastroscopy allows for your doctor to examine the upper gastrointestinal region for signs of illness (e.g., inflammation, aphthous ulcers, strictures, fistulae). As with a colonoscopy, a gastroscopy is less convenient than some other examination methods (e.g., ultrasound) due to the required preparation process in the lead up to the procedure and typically needing to take the day off work. Your doctor needs to have a clear view of the GI tract, so you will be required to fast for at least 6 hours before the procedure.
The procedure generally involves the use of a sedative drug, which means you can expect to have little/no memory of the procedure. Residual sedative effects can take some time to disappear, so you cannot drive yourself home afterwards, nor operate heavy machinery, drink or make legal decisions.