Do you have a PayPal account? If not, sign up for one.

Once you've signed up and put your credit card info in, go to this link: GameStop Gift Card

If you've never used PayPal before, it is very safe IMO. I've been using it for years because it adds protection to your purchases. (e.g., if you have a problem with a merchant, you can file a claim with PayPal and they will handle it for you).