The phrase "gentle arc" refers to a smooth, gradual curve or bend. It suggests a soft and subtle change or movement, without any abrupt or harsh angles. It usually implies a peaceful and calm demeanor. Full definition
Let it run all the way to the left in a big gentle arc until you just clip the kerb on the outside, after which you keep the steering where it is for a couple of moments longer. (evo.co.uk)
His bat met the ball solidly, sending it on a gentle arc into the rightfield bleachers. (si.com)
Nonetheless, in 2002, Samsø erected 10 more turbines, each more than twice as powerful as the originals, arrayed in a gentle arc two and a half miles off the island's southern coast. (discovermagazine.com)