OP, you have two options. Either go with an exchanger on forum with the currrency exchange section, or go with a P2P exchange and find a reputable seller. The rates you'll find significantly higher than spot for any type of gift card, because how illiquid they are, but you just have to live with that.
Either way though, make sure that you are protected via escrow every step of the way, and don't close the escrow until the buyer releases the coins.
Can I transfer my Google Play credit card to Bitcoin please help
Why don't you try redeeming it for cash and then buy out bitcoin? For now bitcoin is not yet acknowledge by google as a form or options for payment. So, the best way to convert that to bitcoin is to redeem it first then buy out bitcoins through exchange. Most exchanges are accepting fiat money to for you to buy bitcoin.
Google play cards can't be redeemed for cash. That's the reason why there is such a huge spread on p2p platforms for these things.