You can get free google play cards by using websites such as
https://grabpoints.com/rewards/free-google-play-gift-codes/ to take surveys, watch videos etc.
The advertisers pay them with cash and when you reach the payout threshold you can either accept payment through Google Play Cards, Paypal or even other Gift cards.
There are many other websites that you can take surveys on and redeem the points for Google Play Cards or other gift cards as well but that is just one of them.