How to Enable Root Account and Enable Username/password Access in GCP

By default, Google Compute Engine offers the browser-based Google Cloud Platform Console tool that lets you manage your Google Compute Engine resources through a graphical interface. Use the GCP Console to manage your resources if you prefer using a user-interface through the browser.

In the GCP documentation setting up ssh keys  which shows how to set up your own ssh key to access all your virtual machines in GCP. here's the summary of steps:

1. Generate your keys using ssh-keygen or PuTTYgen for Windows, if you haven't already.
2. Copy the contents of your public key. If you just generated this key, it can probably be found in a file named id_rsa.pub.
3. Log in to the Developers Console.
4. In the navigation, Compute->Compute Engine->Metadata.
5. Click the SSH Keys tab.
6. Click the Edit button.
7. In the empty input box at the bottom of the list, enter the corresponding public key, in the following format: 
   
   <protocol> <public-key> [email protected] 
   
   This makes your public key automatically available to all of your instances in that project. To add multiple keys, list each key on a new line.
8. Click Done to save your changes. 
   
   It can take several minutes before the key is inserted into the instance. Try connecting with ssh to your instance. If it is successful, your key has been propagated to the instance.