How do I enter a GST input tax amount without a base stock purchase amt since the purchase amount and the associated GST amt fall in 2 different GST filing periods?

Let me provide some details about the GST to help you with how it should be entered, Mliqk.

Goods and Services Tax (GST) is a tax on goods and services sold domestically for consumption. This is the tax included in the final price and paid by consumers. Therefore, a purchase amount is needed for your GST to be calculated. May I know, what you mean when you said about the 2 different filing periods for your tax?

For now, entering the GST TAX on the total amount of the invoice is unavailable. However, if you have multiple line items of products and services on your invoice, you can choose a specific GST amount you've previously set up in QuickBooks by clicking the GST column. 

If you haven't set a tax rate yet, check out this article for more detailed steps: Set Up and Use GST in QuickBooks Online.

Additionally, learn how to view tax reports based on the information collected from the tax invoices posted in the system. I've added this resource for reference: Run Reports in QuickBooks Online.

Keep in touch if you need anything else about taxes or if you're referring to something else. We're here to get you covered. Have a nice day!