I heard Figma is good. I’m not sure if it’s free though. You could try using getpaint.net or Gimp (which is hard to use but it’s very well documented).
Play with gradients in your software of choice, and for the attractive part, I’m not entirely sure. It would depend on your game’s style.
For your purpose, I’d recommend Canva. It’s a free online graphic design platform that makes it easy to create beautiful designs in just a few clicks. It has a wide variety of templates, images, and fonts to choose from, and you can customize your icon to fit your specific needs. It also has tutorials and tips to help you make the most of your design.