When is work original, when does it fall under a license?

I've created a large project containing various services, but one module inspired by a plugin based on GNU licensed code. How can I find out if my work is currently covered by the GNU license?

To say that your work is original, does someone have to read the code base, get inspired, and write their own code without any reference to the original work? Using the naming conventions of the methods from the code base still original, even if the underlying code has been completely rewritten with new logic, new functionality, and the glue that fits in your own code?