No, gold reward coins on an app game could not be considered a form of cryptocurrency. From my point of view, there are two major differences. The first is the very technologies underpinning the two—cryptocurrencies, for example, are built atop blockchain and use strong cryptography for ensuring transactions security. Secondly, cryptocurrency prices are based on supply and demand, while in-game coins are sold at a fixed rate established by the company that developed the game itself. Rather, coins that you've mentioned are a form of virtual currency, and both virtual currency and cryptocurrency are forms of digital currency.