jellybean,
"normally", the initiator uses a (source) port to open the connection
to a (destination) port and the destination will communicate back on
the same (source) port.
Again, "normally", firewalls block only the well known standard ports
for incoming communications (these are the ports on which servers and
services are running which maybe ab/used).
If a firewall allows o/g communications, then obviously an o/g port is
open. The communication in this case will be to a standard HTTP/HTTPS
port on Google's servers (simplified). When Google's servers
communicate back, they will communicate back to the same port.
In effect, "normally", no changes should be required to the firewall -
if an o/g communication with Google's servers is possible.
However, your situation might be different and changes may need to get
done.
SS