You’re right in considering using Eventarc as the best alternative solution. Eventarc will act as a bridge that will allow you to create a trigger that listens to your Pub/Sub topic and forward messages to an internal HTTP endpoint. This endpoint can be an internal IP address or a fully qualified domain name (FQDN), which includes services fronted by an L7 Internal Load Balancer, such as your Kubernetes Ingress.

To implement this, you need to do the following:

1. Create a network attachment: This is a regional resource that explicitly authorizes Eventarc to connect to your VPC network.
2. Configure firewall rules: You need to create a firewall rule to allow ingress traffic from Eventarc to your internal HTTP endpoint.
3. Deploy an Eventarc trigger: This trigger will be configured to listen for messages on your desired Pub/Sub topic and route them to the internal HTTP endpoint exposed by your GKE Ingress.

For more detailed documentation, you can check this article.