I don't think government will come after old transactions even if trading Bitcoin gets ban in future. Any law is applicable from the point of time when it is actually comes into effect. All transactions before that point of time is outside the scope of that law. So there is no problem to report any Crypto profits as CG for filing return of Assessment Year 2019-20.
The actual problem which makes reporting income as Capital Gains difficult is complications in finding Cost of Acquisition. In the absence of COA, one has to pay tax on whole selling value which would result in lots of money flowing out as a tax.
As per law they cannot come after us, what if they come out with a verdict that they need to investigate those transaction for terrorist funding and other illegal activities and with a volatile government like this, we should anticipate extreme things, i am citing this simply because of the past experience we know, anyone expected something like demonetization in the past, i am sure no one in the wildest dream would think about that, your money is not having value and you have to stand in queue leaving everything to get your hard earned money from banks and to see their attitude while doing so, never heard before that you will kill people because of what they eat, we have seen all those in the past five years.
Capital Gain applies only when you have a gain on your invested capital. If you have never bought crypto, there is no question of capital gain either. Make sure to accept the proceed of your crypto sales at a current account. Keep note of all the transactions. Pay tax as per presumptive taxation rules. Remember, if the total volume is over 20L INR, you need GST registration.
I am really ignorant in these things, why would you need a GST registration if there is no regulations about it. I am not sure what is presumptive tax, the head ache of filing the tax is done by my CA, the transactions i got from bitcoin are in my personal account and usually split in my family members account to avoid taxation and i never filed them till now due to the grey situation we have.