If you have no receipt, for all they know you could've purchased it from any store. You could possibly try and find the member of staff who sold the game to you, and ask them to give you a refund.
Otherwise, you're screwed.
You will not get a refund without the proof of purchase which is the receipt.
Although there are some alternatives which you can try. You can give them the exact date and time which you bought the item and giving them the Game number where they scan it would also help. You can also look for the employee which sold it to you.
These guys are wrong all the used games gamestop has have a rating on them on condition as long as you bring it back and ask to talk to the same guy you bought it from he can check it and as long as its in the same condition unless he\she wants to be a dick they will give you store credit and or a new used copy of the game if it did not work. If you want your actual money they will need the receipt though due to the laws game stop have to follow.
one of the people I am a caregiver for works at gamestop