GST apply on any sale of Goods & Services, Doesn't matter how small amount. But if you don't have GST Number you can't generate bills and you can't deposit tax even if you charge it for it you need GST Number for it.
So Unless you are Register in GST you can't create a GST invoice also can't charge any amount of GST to Customer/Client. But now you are thinking now i will not register for GST as I am not register for GST then I don't have to Pay GST. For that
Govt. already decide limit in which you have to Register for GST 40 Lakhs Sale of Goods & 20 Lakhs for Sale of Services. In some State these amount 50% i.e. 20 Lakhs & 10 Lakhs apply. So if you cross limit you have to register for GST. But that is not a only Case if you ---