You can specify multiple GCP resolvers to resolve instances from multiple projects with proper IAM role configuration.
For GCP name resolvers to work, the Project will need compute.googleapis.com enabled.
Gateways will need Read Only Cloud Platform Cloud API access scopes enabled.
How to configure IAM Roles
Create a service account for the project (Project A) where the Gateway is going to be deployed, and copy the email-id of the service account.
Now go to other project (Project B).
Navigate to the roles page.
Create new role with permissions > compute.instances.list, compute.forwardingRules.list & resourcemanager.projects.get
Give it an appropriate name (e.g., ComputeInstancesReadRole)
Navigate to "IAM & admin" > "IAM" page.
Click the "Add" button.
In the "New members" field, paste the email-id of the service account (it should look like a strange email address).
Assign the custom role (ComputeInstancesReadRole) that you just created.
Now you can use the special name resolver syntax to define hosts in your Entitlements.