Is it OK to use credit card on G2A?
G2A PAY is secure payment gateway, so it doesn't matter if you pay with credit or debit card, via bank transfer, or other online methods – you can be sure your financial data is safe.
Is it safe to put my credit card on G2A?
Even if you dont use paypal nobody at G2A will ever see your credit card info (at least they shouldn't). It goes to a third party company who's entire business is dealing with online transactions, its payu I believe.