Follow the Cloudflare Document below.

Restoring original visitor IPs: Logging visitor IP addresses with mod_cloudflare
https://support.cloudflare.com/hc/en-us/articles/200170786-Restoring-original-visitor-IPs-Logging-visitor-IP-addresses-with-mod-cloudflare-

And then, Add 35.191.0.0/16 and 130.211.0.0/22 for CloudFlareRemoteIPTrustedProxy.

<IfModule cloudflare_module>
    CloudFlareRemoteIPHeader X-Forwarded-For
    CloudFlareRemoteIPTrustedProxy 35.191.0.0/16
    CloudFlareRemoteIPTrustedProxy 130.211.0.0/22
    CloudFlareRemoteIPTrustedProxy [your load balancer’s IP]
</IfModule>

After restart httpd, You will see the original visitor IPs on your apache access logs.

FYI: GCP Document of External HTTP(S) Load Balancing Source IP addresses
https://cloud.google.com/load-balancing/docs/https#source_ip_addresses