Assuming the included file is on the same server, and you're not including it with "http://" or the like...
Any variables in scope when you include the file will be available to the included code, the $GET array included. I don't know what you mean by "it tries to get from its own url". If you try to include a file with request variables tacked on, you should get an error. And even if you're including from within a function, the $GET array will be in scope.
In short, the $_GET array and all variables available to the main file at the point of inclusion will be seen by the included code.