There is a real company named Gapbuster, but a lot of scams use the names of real companies. If you were contacted by e-mail by a company with whom you did not register (on their secure website), it is almost certainly a scam. If they ask you to respond by e-mail providing your personal information, it's a scam.
Ask yourself: with all the unemployment, there are many, many individuals available for every job, so why would a total stranger who has no information about me pick my name out of nowhere and contact me to offer me a job? If a company sends you a check, asks you to deposit it and then send some of the money somewhere else, it's a definite scam. If the mystery shopping job pays more than $10-$20, it's a scam.
If it sounds too good to be true, it IS too good to be true.