You can't make a single form publicly accessible -- which, by the way, doesn't mean the Sheet is publicly accessible, it just means anyone can add a form fillout -- even though your Marketo form is publicly accessible? That doesn't make a lot of sense to me. 
Of course the whole thing can be done with an OAuth-enticated session as well, but that's a lot more work for exactly the same amount of real security (if every post goes promptly to the Sheet without any filtering, doesn't matter that you logged into Google first).
You could of course create a separate GSuite account as well.
Also: while Marketo webhooks (by definition) don't support OAuth flow, you can use an in-between gateway to auth to Google and add to a secure Sheet (technically in this case you're skipping the Form, although you still provision a Form-linked Sheet as in the original instructions).
FlowBoost Pro can do the OAuth, but admittedly faster for this narrow purpose is using Zapier. This Zap definition updates a secure Form/Sheet:
Then you point the Marketo webhook to the Zapier endpoint instead of directly to the Google endpoint.
In this case you don't even need to use any mapping on the JS side, since Zapier is doing the mapping. But rather than changing the code, just use the Marketo name on both sides of the map, like:
mktoSheetsTranslateMap = {
'Email': 'Email',
'FirstName': 'FirstName',
'LastName': 'LastName'
}
Hi @SanfordWhiteman ,
I'm using one of your methods to capture the form data into Google Sheets using a Webhook.
Since the beginning, we have seen the problem Error: 401: Unauthorised access to the URL.
I was hoping you could guide me on this one.
Attaching screenshots for your reference.
Hey @Darshil_Shah1 ,
I have already made the Google form public.
Now, the issue is that Webhook is sending only my email address to the Google Sheets.
If I try to register from another email, it's showing an Error: 404 Bad Request URL
From my email:
From different email:
Can you comment on this?
.png)