The problem may simply be that the GRE packets are not getting through.
To resolve this issue, make sure the network router/firewall permits GRE protocol 47 (You will want to find and enable a "PPTP passthrough" feature on your router that knows how to handle the GRE packets). In fact both port 1723 and GRE protocol shoyld be opened/enabled to establish VPN connectivity by using PPTP.
Hope it could solve the problem.