How do I go about recording a GST credit for a stock purchase that does not have GST included in it? Create a new account "GST Credit" and record a "Journal Entry"?
It would be best to first create an GST Credit account to track the amounts for reporting, which would also make it easier in generating the journal entry in the future. When you get to create the journal entry whether you decide to code the amount as Out of Scope, you will see when first creating the journal that the GST field will be empty. If you don't want this amount to have any GST code attached, you can leave this field blank and not select any tax code, then hit save.
From what you mentioned I believe the steps are sound, but of course for peace of mind I would encourage you to reach out to a bookeeper or accountant to see if it is the right steps for your business.