If Initial Response time never runs on a case, then Subsequent Response time rule will never set the case expiration date as it is expecting that field to be populated by the Initial Response Time Rule. Remove this expectation.
To Test: 1. Create a case in the CSP 2. Immediately add a public comment as a technician and set case status to Waiting on Customer. (This needs to be done right after creating the case and before GK runs and sets the Initial Response Time) 3. Verify Gatekeeper runs after above comment and Time to Service Breach is set correctly