I am not sure that there is any "virtual IP" capability in GRE. But I have impelmented something that I think may be close to what you are looking for. At a customer network we have many remote sites where we wanted to provide a sort of high-availability capability. We run 2 GRE tunnels from the remote to 2 router at the main site. Both tunnels come up and active. We run EIGRP over the tunnels and manipulate the metric so that we have a primary and a backup. Normally traffic flows over the primary. If there is any kind of problem with the primary connection (could be on the main router, could be in the network connecting them) EIGRP will detect loss of connectivity and quickly and smoothly transfer traffic over to the backup tunnel. This is the kind of automated and transparent functionality that you usually look for with virtual IP functions. It has worked well for us.
I do not understand your comment about the remote site does not support 2 simultaneous GRE tunnels. If you mean that they are not willing to do 2 tunnels that is one thing and probably not much that you can do. But if you really mean that the remote does not support (is not capable of doing) 2 GRE tunnels then I am puzzled.
We are pleased to announce availability of Beta software for 16.6.3. 16.6.3 will be the second rebuild on the 16.6 release train targeted towards Catalyst 9500/9400/9300/3850/3650 switching platforms. We are looking for early feedback from custome...