The following requires, as James Hirschhorn pointed out, the Professional verison of PyCharm.

1. Assign a public IP to the remote machine on GCP.
2. Run gcloud compute config-ssh to automatically add the VMs of your project to your ~/.ssh/config or manually add the public IP of your VM to it. If you skipped step 1. then you have to run gcloud compute config-ssh every time you re-start the remote VM, because it always gets a new IP assigned. The ~/.ssh/config gets populated with many entries in the following format:

   Host the-vm-host-name  # use this in PyCharm's Host field
       HostName 123.456.789.00  # the VM's IP address
   

3. Use the Host name of the remote you want to connect in your Deployment configuration in PyCharm
4. Add a remote interpreter: select the remote server from the drop-down (the one previously created) and point PyCharm to the executable python of your Python installation.
5. Done

It's fairly easy to accomplish. You need:

1. PyCharm Pro

2. Create and format SSH keys

3. Config your Compute Engine instance with the SSH keys

4. Configure PyCharm

You can follow this tutorial that I wrote.

My understanding is that you need the Pycharm Ultimate Edition to support remote servers. If you have Ultimate, then you can follow these instructions.