GStack: TIdStack = nil;

GStack is a global variable that represents the current stack object for the Indy component set. GStack is assigned a value by calling the IdStackFactory function when the Indy library is initialized. 

GStack should not be changed; it depends on the platform hosting the Indy application and the class reference indicated in GStackClass. 

GStack can be used in applications to access useful low-level properties and methods of the protocol stack.