GFContentLength = -1;

GFContentLength is an Integer constant that specifies the default value for the TIdHTTPResponseInfo.ContentLength property. This is used in the class definition and the constructor. 

GFContentLength is the value assigned to a TIdHTTPResponseInfo instance, and used when writing headers for an HTTP response. GFContentLength is the value used to ensure that content-length headers will not be written in the HTTP response.