Switch bootcamp partition from MBR to GPT?

A bit of background into what's been happening so far:

• Trying to download Windows 10 onto my Macbook Pro (mid-2012, OS Sierra 10.12) through Bootcamp. Everything downloads perfectly, partition goes up perfectly, it's all good.
• Open Windows 10. Get error for "no bootable device found" despite the thumb drive I used still being plugged in.
• Reboot computer, boot from EFI boot. It works!

So now I've finally booted into Windows 10, but when it gets to the part of the installation where I'm supposed to select the part of the disk I want to install Windows on, when I select the bootcamp partition it says that I can't install Windows on an MBR disk, because it requires a GPT disk instead.

And I have no idea how to switch to a GPT disk.

Pretty much everywhere online says to download the software "gdisk" but I am highly wary of downloading any sort of software from the Internet, so I would prefer to not go that route.

I also saw some advice where someone said to format the partition to "Mac OS Extended (Journaled)" but when I did that, it said that it couldn't download to a disk that wasn't "NTFY" (or NTFS..... honestly, it was 1 AM at this point and my memory is a little foggy).

I've been trying to look for solutions for a solid day, but none seem to be fitting to my particular problem, and I've got no idea how to solve it. Any help would be greatly appreciated.

I've included a screenshot of my specs, if they can help: