So I think I found a way of doing this. You can just update the file google_cloud_credentials.conf on
$SPLUNK_HOME/etc/apps/Splunk_TA_google-cloudplatform/local with the new value according to the docs, such as:
[<name>]
google_credentials={"type": "service_account","project_id": "my-project","private_key_id": "32a3be8f2f0dcfe967ea558e486deaereacfas0c2497e"}
After you do that, the passwords.conf file will be automatically updated with the new values as soon as you go into the Configuration page on the Splunk Console. Restarting Splunk service also works if you wanna keep in the command line.
Another option you have is forget about passwords altogether and use the GCE service account. Take a look at this post: https://answers.splunk.com/answers/774312/use-gce-service-account.html
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