I am sure you have seen plenty of times these gradient-looking glosses on websites, banners and all sorts of buttons. They are there, they make objects shiny and don’t stand out too much.
Then you and lied to yourself “Ha! I can do this myself in a snap, it’s easy as pie”. Then, you used a large, soft brush, you painted a large white spot, erased half of it and rotated it 45 degrees.