And, interestingly, I had already had that "Send all traffic over PPTP tunnel" unchecked. But I am still having the same problem that sota767 originally described. I'm on a 10.6.x Macbook Pro.
sota767, could you post the interface configurations of ppp0 and gif0, and what you did to manually configured gif0 after bringing up the PPTP VPN connection? That was I can compare it to my configuration.
I am, as you were, able to ping across the PPTP tunnel, but not to the opposite end of the IPv6 transit across gif0. I have tried setting the "ifconfig gif0 tunnel" with the client address (which is static, and successfully configured on my ppp0 interface), and I've tried both the "correct" address for the server, and the other-end address of the ppp0 point-to-point. Which of those server addresses are you using on the "gif0 tunnel" command?
Maybe if I can just see the commands you are running to configure the gif0 interface, that might be enough. The ppp0 interface is pretty much handled by the Network preferences panel, and I think is right.
And, as noted, I've already turned off the "Send all traffic through" checkbox for that VPN interface.