Dr Gagner is known for his contributions in the field of Minimally Invasive Surgery, in particular the first description of laparoscopic adrenalectomy for Cushing syndrome and pheochromocytoma (1992), first description of laparoscopic pancreatectomy (distal and proximal) (1992-93), first description of endoscopic neck surgery with parathyroidectomy in 1995, first transgastric cholecystectomy in 1997 (NOTES), first description of laparoscopic duodenal switch for obesity in 1999 and laparoscopic sleeve gastrectomy in 2000. More recently, he was co-surgeon for the first World transatlantic robotic cholecystectomy, between New York City and Strasbourg (France), the only surgical article published in Nature in 2001.

He has over 250 published journal articles, 40 book chapters and 6 books on Minimally Invasive Surgery. He has been visiting professor and operated in over 60 institutions in 48 countries. He has held prominent positions in more than 35 societies and organizations. He has been on the editorial boards of 12 surgical journals.

Dr Gagner also has received a number of honorary memberships and awards throughout his career. His most recent contributions focus on innovative upper digestive tract surgery like bariatric and endoluminal gastric procedures.