Minecraft – When I buy PC Minecraft at Gamestop, do I get a code to get Minecraft

First make sure that you're logging in with your email address, not your old Minecraft username. Everything uses your email address now that you've migrated your account.

If that's not the problem, this is simply an account bug that we can't help with. For account migration issues that aren't resolved by that help article, you need to contact Mojang support as it says at the bottom.

(No, accounts can't be un-migrated.)

No, Minecraft does not have any form of micro transactions (buying weapons/cosmetic items). Gift cards are solely for obtaining the game.

Besides, part of the fun of the game is making the weapons.