Can't connect to RDP from Compute Engine
You should first check that port 3389 is open on the GCE firewall. If not, run a command like this to create the rule $ gcloud compute firewall-rules create rdp --allow tcp:3389 --source-ranges 0.0.0.0/0
. More info is available here.
Also, make sure you have the right username and password. This was set when you first created the instance. To check, please run $ gcloud compute instances describe INSTANCE_NAME --zone ZONE
and it will give you the initial user and password.