I can query the log explorer to identify my errors I want to alert against:
Hello @david_rise4,
Welcome to Google Cloud Community!
You mentioned that you already configured a log based alert with an email notification. You may find it in this document using the JSON payload.
If you want to apply this for the textpayload, you might want to use the label extractor key log.extracted_label.KEY. From the cloud console, go to Monitoring>Alerting>Policies to download the policy definition as JSON then add your label extractors. See this link for your reference