You need to follow the following process:
1)Firstly, open the browser and search for cloud.google.com
2)Navigate to my Console
3)Select your Project name.
4)Navigate to Networking > VPC network
5)Click on "Firewalls rules"
6)Click "Create Firewall Rule"
7)In order to apply the rule to select VM instances, choose Targets -> "Specified target tags", and then you will enter "Target tags" (the tag name). This tag will be used to apply the new firewall rules onto an instance of your choice. After then, do make sure that the instances have the network tag applied to them.
8)In order to allow incoming TCP connections to port 9090, in "Protocols and Ports" enter tcp:9090