Unless I'm mistaken, it seems to be impossible to run OSPF3 over a GRE tunnel. The GRE interface shows up as "inactive" in /routing ospf-v3 interface print. My understanding is that this is because it doesn't have a MAC address so the OS doesn't generate a link-local address for it. I can't see a way around this (trying to add a link-local address manually fails), but I'm hoping that someone here will prove me wrong... I'm running RouterOS 5.14.
Otherwise, the GRE interface works, in that I can ping the remote end's IPv6 address.
I haven't properly tested this, but I think the same would happen with a GRE6 interface.