If your Gravely self-propelled lawn mower will not move at all it is most likely due to a worn or broken belt. If it is broken or shows sign of wear then it should be replaced. If the belt still looks good, then it might have just come off of the pulley and needs to be put back on.
If the belt checks out fine, then check the drive cable for wear or to see if it is broken. Again, if it is then it would need to be replaced. Also, check the wheels t to see if they have traction left on them, if they are worn out then the lawn mower will not have good traction and may not be able to move the mower. If they are worn out you should replace them.
You can find the model and serial numbers located on your Gravely push or self-propelled lawnmower on the rear left of the frame by the engine.
NOTE: if you need a part for the small engine in your Gravely push or self-propelled lawnmower, there will be a separate model/serial number for the engine. Here are the small engines that are available in your Gravely push or self-propelled mower. Visit the page of the engine you have to find out where the model/serial number is located for your small engine model: