Nowadays the best investment is that you hold that amount in your wallet as in the next month the halving will occur and with that the price will increase much faster and o a higher value.
Not sure on how you can use the $20 worth of Bitcoin to make more exact for trying to gamble with them. I do not suggest that you gamble with them because you will lose.
Try to get in on some campaigns to earn you free Bitcoin you can then try to get more Bitcoin.