Otdfbt

I want to setup an IPSec VPN from a remote ISP Datacenter to my server located in Google Cloud through his external IP (Not internal IP of server).
Is it possible?
All documentation on Google I found is about IPSec VPNs with encryption domain based on Private IPs (local & remote ).

Here is what I want to setup:

 Remote_Server
 "Public IP"
 |
 |
 "Public_IP_peer"
 [Remote_ISP_Peer]
 |
 (IPSEC_VPN_through_Internet)
 |
 "Public_IP_Google_Cloud_VPN"
 [Google_Cloud_Platform]
 |
 |
 "External_IP_Public"
 Local_Server_in_Google_Cloud

GCP IPsec VPN supports accepting the route with both public IP address range(CIDR block) and private IP address range(CIDR block). You are free to use both kinds of IP address range(public and private) to point to your on premises networks.
For the detail VPN config please refer to 1

GCP can accept the route with Public IP address range from Cloud VPN IPSec tunnel. That means in Google Cloud you can access your servers on premises through Public IP addresses via VPN IPsec tunnel. But if you want to access VM instances in Google Cloud through VPN IPsec tunnel, you can only access the VM instances via the private IP addresses.
Yes, for VM instance, only internal IP address attached to its network interface. The External IP address only using for communication with the Internet. Please refer to Google public docs for the different IP address range in Google Cloud.

For your case, I can figure all two solutions:
You can get several public IP addresses from you current ISP, and assign the public IP addresses to the VM instances in Google Cloud and advertise the Public IP addresses back to you ISP environment through the VPN IPSec tunnel
You just keep using the private IP addresses for your VM instances in Google Cloud, and only advertise the /32 route to your ISP environment, it will have very low risk for getting the IP address conflict.