The reason for not tranquilizing it is tranquilizers will take between 3-15 minutes to kick in and have a chance to highly agitate the animal or make it aggressive until it goes down.  A gorilla can easily rip a man's head and arm off his body with very little effort. Imagine a toddler. And imagine the mother of all lawsuits and trauma if the gorilla didn't take well to the tranquilizing shot and ripped the kid to literal pieces. And any tranq shot that could put a gorilla down in less than 30 seconds would paralyze and seize his lungs instantly. Which would make it suffer and die nonetheless with one last chance to freak out. Best case would have been gorilla not being interested in the kid and wandering back to its cage but since it stayed in contact with the child they had to put it down instantly.