I'm trying right now to replicate an existing GCP project set-up in Pulumi. In GCP console you can add notification channels to Error Reporting via GUI like:
In Pulumi, I found out how to create a notification channel (link to the docs). Now, how can I add my created notification channels to GCP's Error Reporting via Pulumi?

As per the Pulumi doc and by using Pulumi you can only create notification channels and cannot add this notification channel to the Error report using Pulumi. The only way to do this is using the GCP Console.
In GCP Console go to error reporting page, Click on Configure notification as shown in below Fig(i)
Post clicking on this you see a fig(ii) as shown above. Click on the notification channels and select the notification channel which you created using Pulumi as shown in below fig (iii). This will add your notification channel to Error reporting.