GCP API Gateway Static IP for egress

Question

I have deployed an API Gateway on GCP. I would like to attach a static IP to the gateway so that I can add the IP to an allowlist of another service, that the API Gateway forwards requests to. The service is a third-party service, that only accepts requests from IPs on their allowlist.

I have set up a load balancer for the API Gateway, which does have a global static IP attached. However, this does solve the egress traffic, I see in the logs that the IP isn't static. Is there a way that I can attach a static IP to the API Gateway for egress, so that I can add a single IP to the other service?

Any help on this would be appreciated!