OpenSCAD is a programming environment which allows programmers to create 3D models using a programming language. This is very clever, but for most of us it's going to take too long to learn enough to make anything useful.
If you download the Gear Library and install OpenSCAD, you will get an interface that works with the library to create just about any gear you like simply by filling in the various drop down boxes.
Anyway, once the model has been created, you can save it as a .stl file to 3D print it. At the moment I can't see a way to save that as a .step file which would be much more useful.
Stop Press... you can convert .stl to .step files by using other programs such as FreeCAD but I don't think this is very satisfactory.