You are open to the internet and are being attacked. However, the attack is likely not
directed at your VPN server, although the errors you are getting are probably from
its firewall.
The
Generic Routing Encapsulation (GRE)
is defined by Wikipedia as:
Generic Routing Encapsulation (GRE) is a tunneling protocol developed by Cisco Systems that can encapsulate a wide variety of network layer protocols inside virtual point-to-point links or point-to-multipoint links over an Internet Protocol network.
This is a versatile method of passing messages from all sort of other protocols
to the recipient. This attack is usually directed at routers with the intent
of establishing a tunnel between the victim router and the attacker,
that will allow the sniffing of data and more.
Not all the possible uses of GRE as attack vector are known even today,
since it can carry multiple types of payloads from other protocols than just TCP/IP.
GRE as attack payload became popular with the Mirai botnet but was relatively
less-known before that.
But to answer the question, although your VPN server is perhaps not being
attacked, IKEv2 is considered to be among the fastest and most secure
protocols available.
Security depends mainly on the strength of the login password.
You may find some in-depth discussions of GRE attacks in the following references:
The first link offers some advice for the mitigation of GRE attacks,
although perhaps more related to CISCO routers.
For someone who is not a security expert or doesn't have the services of one,
what you can do is ensure that the firmware of your router(s) is well updated.
Review also all the settings of the router, with a view to disallowing
everything that is not absolutely essential, including GRE if you can.
Rebooting the router regularly is also a good idea.
As your IP segment on the internet is being attacked,
you should do the same to the firewall of the VPN server or any other device
that is exposed to the Ethernet.
