Excessive amounts of vibration on a Gravely zero turn radius lawn mower is usually due to a loose or bent blade. Make sure to tighten the bolt holding the blade in place. If the blade is bent then it would need to be replaced.
Always make sure the lawn mower is off and the spark plug is removed before doing a repair on the lawn mower. Use caution when dealing with the blades so that you are not injured.
The model and serial numbers on your Gravely zero-turn mower can be found on the main frame and are six digits in length beginning with the number "9".
NOTE: if you need a part for the small engine in your Gravely zero-turn mower, there will be a separate model/serial number for the engine. Here are the small engines that are available in your Gravely mower. Visit the page of the engine you have to find out where the model/serial number is located for your small engine model: