How can I determine how much "gains" I've had from Bitcoin?
It may be a bit of work, but you should be able to trace on a blockchain explorer (eg blockstream.info) when you acquired the bitcoin with the current addresses you have.
If you can't prove how much you paid for it with a record for each purchase, your tax jurisdiction may accept a historic exchange rate from a well known and reputable public exchange at that time. I suggest asking your government if that is a possibility first, before reporting that you received the coins for free.
To calculate your gains from your purchases and sales, different accounting methods exist (FIFO, LIFO, WAC, etc). If your government is not clear on which method they accept or don't know how to do that, you may need to contact a local tax advisor to help you out further.