Here's a sample project that uses Managed GDI+ to draw several lines and an ellipse on a form, and fills the ellipse with a gradient background:

If you don't want to download and run the project, you can just look at its C# code.  Note that GDI is generally considered too slow for games, where Managed DirectX is more often used.  Managed DirectX is available as a free download from Microsoft but is not installed by default at present.