Hi Mauro!
Have a look at our finished example:
https://mdn.github.io/learning-area/css/styling-boxes/letterheaded-paper-finished/
Inspect the circle in browser developer tools, and you’ll see that I’ve done this using a combination of border-radius and box-shadow.
For the top to bottom gradient, again look at my example - I’ve used multiple background images to attach all of the images, and specify a top-to-bottom (“to bottom”) gradient that covers all parts of the gradient.
In terms of helping others - please do! I am always very busy and answering all the questions takes up a lot of time. If you get stuck, feel free to ask for my help!
And I love the guitar playing, thanks for sharing! Here is a rough demo of what my current band sounds like: https://www.youtube.com/watch?v=JAcFozYUHUU