Your question can be generalized as how to "convert: a CAD model to G-Code (or NC Code).
Generally CAD is used to design parts and CAM (Computer Aided Manufacturing) software is used to design the manufacturing process for the part designed in the CAD tool.
In some cases this is highly complex (e.g. for 5 axis milling) and in some cases this can be simple (e.g. your case of following a contour). If you have a 2D image there are a number of free tools you can use to export contours these as G-Code, like:
These are similar in nature to a CAM software, but they lack certain functions and so, they are not ready for series production, but are OK for prototyping and experimenting.
There is a collection of free tools listed here, but I am not sure about the quality.
After you have generated the G-Code you can visualize it in NCViewer. This also might help to improve your G-Code skills.
Please note that I am not including plasma or laser cutting generally in the above description, as these might require some other advanced features for series production, like nesting.