Obtaining certain classes of license is not mandatory in GT5 in contrast the fact that it has been due in other series of GT, so you can take part in the events at least if you can meet the minimum requirement of A or B-spec levels necessary for participation or qualification.
However, not having to do any licenses means the essence of being successful as a simulative racing driver is awfully messed up unlike in reality where getting a driver's license is indispensable for gaining experience as the calling, therefore, people's preferences toward this alternative system differ vastly since it's still vague whether this feature is suitable in a real driving simulator or not.