Then, you either assign a public IP to each machine, or build yourself a solution...luke a NAT gateway. Google does not offer a service like this.
The machine that are part of the same subnet cannot talk to each other directly.. they have to go via the google gateway. This reduces the posibility of implementing High Avalability solutions that are based on broadcast or Virtual IP (MS SQL, Windows failover, vrrp...etc).
Labels that you apply to VM's or firewall rules are free text without the possibility to select from a labels list/cloud or type a new one...accidents can happen.
The MySQL solution is only reachable via INTERNET!!!. They seems to have a beta for using a local socket if you install a sql proxy on each server where you need sql db access...but not in production.
No support for MS SQL.