How to know which user created the GCE Instance
Note: I'm not a GCE user yet, the answer is based solely on documentation.

You could be Viewing Audit Logs in the Google Cloud Console, more specifically the Admin Activity logs:


  Admin Activity logs contain log entries for API calls or other
  administrative actions that modify the configuration or metadata of
  resources. For example, the logs record when VM instances and App
  Engine applications are created and when permissions are changed. To
  view the logs, you must have the IAM roles Logging/Logs Viewer or
  Project/Viewer.


You would be looking for the User identities in audit logs:


  Audit logs record the identity of the user performing logged actions.
  The identity is held in the AuthenticationInfo field of AuditLog
  objects.


Ideally you'd be looking for the GCE instance creation events, but some/all of those may be too old for the logs retention policy. Then maybe search for VM instance start or other relevant events.

You could also check and maybe cross-reference the audit logs info with that from Usage Reports or exported billing info.

If this is a regular, on-going activity you may want to setup an automated log exporting and processing pipeline.