The short answer: No, GameStop does not routinely test every used game for functionality before putting it on the shelf. Here's why—and what it means for you as a buyer.
These games are tested and guaranteed to work. They carry a slightly higher price tag but come with a more robust warranty. Look for the "Certified" label in-store or online.
Games with deep scratches, cracked cases, or obvious water damage are typically rejected at trade-in or pulled from the shelf if discovered later.
Ultimately, the responsibility for verifying a working game falls on the buyer. Use the return policy if something doesn't boot.