A worker at gamestop ripped me off?

Ok so I had a brand new $50 dollar gift card and spent $10 on a game for my bro and he swipes it and says there's $9.21 on it i say there's supposed to be $50 and he said nope, so I give him a $5 dollar bill and he takes the card and puts it somewhere. I went home and checked amazon to see if I used it and i didn't, I can't even do anything really because the proof is the card and he has it. Worst part is I went in there for a Minecraft PC gift code and he replied "There's no such thing as a PC money card" so I leave with no Code and a $50 gift card useless. I wanted to start an argument but that wouldn't have gotten me anywhere.

Added (1). it wasn't an amazon giftcard it was a visa giftcard

You can't use amazon gift at a gamestop store
amazoin gift card are only for amazon online products

Alright, first off, please make this a question and not a complaint. Second, whoever was working the counter clearly is either an idiot at computers, or he didn't understand you, because I have seen Minecraft PC accounts that you can by at stores. I can't tell you anything more because you didn't specify where you got the gift card, and that's half the equation.

Cancel the card and call the headquarters.