You need to:
Go to cloud.google.com
Go to my Console
Choose your Project
Choose Networking > VPC network
Choose "Firewalls rules"
Choose "Create Firewall Rule"
To apply the rule to select VM instances, select Targets > "Specified target tags", and enter into "Target tags" the name of the tag. This tag will be used to apply the new firewall rule onto whichever instance you'd like. Then, make sure the instances have the network tag applied.
To allow incoming TCP connections to port 9090, in "Protocols and Ports" enter tcp:9090
Click Create
I hope this helps you.
Update Please refer to docs to customize your rules.