Currently hostnames cannot be used as VPN endpoints in GRE. However, Teltonika routers supports many other VPN protocols.
If you just need VPN tunnels with dynamic IPs and encryption:
- OpenVPN also uses encryption and can support DDNS (more information here)
- If you specifically want IPsec, you could use L2TP over IPsec (more information here)
By the way, could you please let me know why you need GRE over IPsec with dynamic Public IPs?
Best of Luck,