Your GRE interfaces have local /32 addresses. And you don’t have any route pointing over the vpn tunnel.
So no packet will ever be routed over vpn
Add interface route onto gre tunnel, or adjust gre tun0 addresses to be in the same /30 subnet.
Moreover, ipsec remote/local tunnel addresses should be local and remote gre tunnel addresses.
remote 203.0.113.2
source-address 192.51.100.2