You can always use the SSH link on the cloud page to connect to your VM, however that user name is what’s your Google account name. Below I’m showing how to use Putty to connect to the same instance with a different user name.
1. Use PuTTY Key Generator to create your own private/public keys, make sure to provide your user name in “Key comment” field
2. Copy the public key to your VM’s public key section
3. Use Putty to connect to this VM by using the corresponding private key.
3. When prompted, type in a user name that you specified in step 1 above
4. all good to go now!
p.s.
After your enabled the public key(step 2 above), Google will create this user on VM and gives it sudo access.
Refer to below on how to access a postgresql on GCP.
https://www.linode.com/docs/databases/postgresql/how-to-access-postgresql-database-remotely-using-pgadmin-on-windows/