After looking around a bit it looks like there are some gotchas with running Freeswitch on a cloud provider. So far there is official documentation about running it on Amazon EC2, and it says it's not officially supported. I'll try to adapt it to Google Cloud Platform, maybe this will work for you.
1 - Create a Debian Jesse 8 instance
gcloud compute instances create freeswitch-test --image-family debian-8 --image-project debian-cloud --tags=freeswitch
2 - Create the required firewall rules to open the ports it needs to run. From the documentation looks it should be UPD:16384-32768,TCP:8081-8082,TCP/UDP:5060,UDP:4569,TCP/UDP:8000
gcloud compute firewall-rules create freeswitch-policy --allow UDP:16384-32768,TCP:8081-8082,TCP:5060,UDP:5060,UDP:4569,TCP:8000,UDP:8000 --source-ranges=0.0.0.0/0 --target-tags=freeswitch
Then you want freeswitch to be able to know its external IP when it starts up. I think the best way might be to reserve a static IP and then create a forwarding rule:
gcloud compute addresses create freeswitch-ip --region us-east1
gcloud compute target-pools create freeswitch --region us-east1
gcloud compute target-pools add-instances freeswitch --instances freeswitch-test --instances-zone us-east1-b
gcloud compute forwarding-rules create freeswitch-forwarding --address freeswitch-ip --region us-east1 --target-pool freeswitch
Now to configure the static IP on freeswitch:
conf/vars.xml
<X-PRE-PROCESS cmd="exec-set" data="bind_server_ip=[YOUR-IP]">
<X-PRE-PROCESS cmd="exec-set" data="[YOUR-IP]"/>
<X-PRE-PROCESS cmd="exec-set" data="[YOUR-IP]"/>
conf/sip_profiles/internal.xml
<param name="aggressive-nat-detection" value="true"/>
<param name="multiple-registrations" value="true"/>
<param name="ext-rtp-ip" value="$${external_rtp_ip}"/>
<param name="ext-sip-ip" value="$${external_sip_ip}"/>
<param name="NDLB-received-in-nat-reg-contact" value="true"/>
<param name="NDLB-force-rport" value="true"/>
<param name="NDLB-broken-auth-hash" value="true"/>
<param name="enable-timer" value="false"/>
<param name="auth-calls" value="true"/>
conf/sip_profiles/external.xml
<param name="aggressive-nat-detection" value="true"/>
<param name="ext-rtp-ip" value="$${external_rtp_ip}"/>
<param name="ext-sip-ip" value="$${external_sip_ip}"/>
<param name="NDLB-force-rport" value="true"/>
conf/autoload/switch.conf.xml
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<param name="rtp-start-port" value="16384"/>
<param name="rtp-end-port" value="32768"/>