GCode is the output from CAM software - it's usually a text file that looks pretty weird when you open it. It contains the instructions for your mill. Different mills expect this code to be in different formats. When you export GCode from Alibre Workshop, simply pick your mill and the output will be massaged so that your mill can accept it.
Alibre Workshop works with most any mill. If you don't see yours, let us know and we can probably add it pretty fast.