gcppubsub.yaml was removed and replaced with https://github.com/google/knative-gcp (more info)
The last available version of gcppubsub.yaml is in v0.8.2:
https://github.com/knative/eventing-contrib/releases/download/v0.8.2/gcppubsub.yaml
Since this sample (https://knative.dev/docs/eventing/samples/iot-core/) is not using content from v0.10, it should either be updated or removed (remain as an archive in the v0.8.x docs).