SOLVED: Magento 2 GET 500 Internal Server Error

Internal Server Errors and/or Gateway Errors can almost always tracked back to one or another PHP error. Turn on your logs and see what you can find from there.

You need to reset the permissions on your media folder and files.

run this from your Magento installation folder

find . -type f -exec chmod 644 {} \;
find . -type d -exec chmod 755 {} \;

If this fixes the problem you should then review the permissions of your Magento installation according to best practice. Further info here What permissions should I give /var and /media?

Also enabling error reporting within php will give you further information to the exact nature of the error instead of the unfriendly 500 Internal Server Error.