There is no criteria or metric that will tell you if you gathered all the necessary requirements or not. The only way to know that your requirement gathering is complete is by using your common sense. Did the client say everything that he needed and wanted about the project? Do you COMPLETELY understand your client's business model?
You have to be honest with yourself in answering the above questions, because if you have the slightest hunch that you still need more information, or the client may need other features, then you should hold another discussion with the client, until you feel that you can see how the project will be executed from start to finish, and you'll be able to see the full resulting product based on your requirements at the end of the finish line. Only when you "feel" this way is when you've gathered all the requirements that you need, and you can start the project.