A documentação parece bastante clara:
... Source IP addresses for browser-based SSH sessions are dynamically allocated by Cloud Developers Console and can vary from session to session. For the feature to work, you must allow connections either from any IP address or from Google's IP address range which you can retrieve using public SPF records. ...
Esse link explica que os intervalos de endereços IP atuais usados pelo Google são armazenados no DNS e podem ser recuperados da seguinte forma:
nslookup -q=TXT _netblocks.google.com 8.8.8.8
nslookup -q=TXT _netblocks2.google.com 8.8.8.8
nslookup -q=TXT _netblocks3.google.com 8.8.8.8