Hello Kelvin,
According to Wikipedia, mainland China blocks all GCP services except to Hong Kong, Macau, and Taiwan (source:
https://en.wikipedia.org/wiki/Google_China). That said, testing actual VIP URLs that resolve to GCP IP addresses can be performed after the migration to GCP begins.
This KB article will be updated with the latest available information.
@Kelvin Wong, if you are involved with this testing, please keep this post updated with your findings.