who can teach me to prepare ubuntu for ssh root access by putty in gcloud VM

i created a VM in google cloud through gcloud console with Ubuntu18 i can connect to root SSH through gcloud console web-based SSH but i can not access this server by putty in windows i created ssh keys and config all based on google help documents and did all correct, but i can not connect to that server Port 22 is open in gcloud firewall rule i edited ssh_config file and add permitrootlogin without-password but i still can not connect to ssh by putty Although when i input server ip in putty and click on Open, then terminal window open and ask for username. but when i enter any username such as root then i get error such as this : "publickey" i did config centos7 VM hardly and now i can connect that by putyy without any problem but NOT for ubuntu18 now i want to know that how i can connect to this server through ssh by putty