For all your gar knows you could be a heron coming to eat him. That's why he gets startled. This basic reaction to potential danger is hard wired into them through evolution.
How many of us have to be careful going near the tank in case our prized fish goes nuts and beats himself up. A lot of the thinking behind bare tanks is that fish have nothing to injure themselves on when they occasionally freak out. Some fish are worse than others. I have tinfoils, balas, cigar sharks and a chalceus which are all very skittish.
And on the other hand, some fish are as docile and placid as little puppies. I doubt very much you could train a very skittish fish into being a little puppy.