Here's a link to the article on the official wiki: Game Cards - Top Eleven Official Wiki

"Game Cards are a way to recieve Tokens, Health Packs or ingame cash, depending on the code. Codes can't be obtained anywhere at the moment, but they will be used in promotional campaigns, competitions, when purchasing certain products (for example, like how you can redeem codes from Pepsi products in Turkey at the moment) and as purchasable gifts (you buy a code and send it to your friend as a gift card, for example)"