I was given a Costco gift card with a zero balance, can I still enter and buy stuff at 5% markup?

No, but you could buy a membership or at least you can get something at the food court.

Sarcasm? That person really loved you😱

No but you can get someone to buy you another one so you can go shopping with it

No

No you got to be a member

Call store and ask....fastest way to get correct answer

You can't buy stuff with a gift card that has a zero balance.