While technically it could be called gambling, it's definitely not in the same league as risking real money on a bet. With virtual currency, that has no exchangeable value, you can never judge the quality of your opponent and their betting patterns almost become meaningless - so it's hard to improve or derive practical knowledge if your intention was to convert to paid betting later. As the person placing the bet, you will be more likely to take time and think through your betting strategy when real money is at risk.