As Plater said,

You need to put properties inside your struct. The gridview binds to public properties, not fields.

Code:
//Struct to be sent
public struct UserDataSent
{
public string userID , fName , lName , IMO , status, businessAddress, city, state, zip, telephoneNumber, mail;

public string UserID {[INDENT]get { return userID; }
}

...

}
...