Why do you want a google play card? How does it work? You cannot use it from the hushed website, which asks for either a card or bitcoin.
You can top up via the hushed app, which bills your google / itunes account. But, the way I understand it, if you buy a google gift card, you still have to redeem it by adding the balance to your google account. If you use the app from your main phone, with a google account linked to your real identity, this means that the gift card purchase won't show up on your credit card statement (assuming you paid cash) but the company running Hushed will still get your real name.
You could have a secondary phone with a google account in the name of John Doe and link the gift card to that, but that would defy the key advantage of Hushed, which is to use your main phone and not carry around a second one.