Never ever run Facebook Ads without entering your proper GST no. in payment setting, doesn't matter for what purpose you've been running Ads on Facebook. Even if you're spending 10-20K+ Rs. on facebook put in your GST first.
There are so many reasons for this. And I'm telling you one of the most important reason here...
if you wont put ur GST details correctly in FB and run ads, FB will charge you GST anyway and because you've not entered your details there all the tax amount you've paid to fb will go in bane. You won't be able to take GST credit ever and believe me 18% amount is too big to loose for any marketer.
Even though you don't do business in India (like Dropshipping) or you're not purchasing anything for which you can reimburse GST credit still you need to put in GST details otherwise all the amount you've spent on Facebook won't count in your business expense and it will lead you to pay a HUGE and unnecessary Income Tax.
This is just a tip of the iceberg, it's a deep subject in itself but I know this can be complicated for some friends in here so I would suggest you to talk to your CA about this, if you don't have get one asap, even if you're doing only 5-10L revenue.