can't speak to GCP specifically but usually the issue is they are host-attached and can't be migrated, so need to be wiped on VM termination or migration -- that's when you lose data.

Reboots typically don't otherwise do anything special unless they also trigger a host migration. GCP live migration has some mention of support though

GCP mentions data persists across reboots here https://cloud.google.com/compute/docs/disks/local-ssd#data_p...

note that stop/terminate via cloud APIs usually releases host capacity for other customers and would trigger data wipe, a guest initiated reboot typically will not.