Generate and download a gear you need for your project. You can afterwards 3d print the gear for your diy project or use it as a placeholder part in an assembly in digital form.
Make sure that the gear meets the requirements of your project before use, especially if it is going to be 3d printed. You should take into consideration, that 3d printed gears are going to be far weaker and less precise than traditionally manufactured ones. You are only allowed to use gears generated with this app, if a failing gear is not going to lead to a safety hazard or break other parts.