Can't really make that assumption, on the GPT side. I have bought two 2TB computers and one 3TB external, powered, hdd in the last three years, and none of them shipped as gpt-formatted, I had to do that.
Wiz, every UEFI based computer I've had the opportunity of working on since the release of Windows 8 (which had that OS installed by default), has had GPT partitioning. One reason why? Everytime it gets upgraded, a roughly 450MiB partition is created. With Windows 10, for those who bought (or upgraded) early, some has many of these small partitions, as the OS has had several major upgrades.
It's the major ones (example, the Anniversary Upgrade) that creates a new 450MiB partition, one of mine has three, plus a couple from the 8.1 install. While that may not be the only reason why these drives are formatted as GPT, it's one. The minor upgrades may not create another. though it can't be ruled out that it won't happen.
Another is that UEFI/GPT goes together, just as BIOS/MBR does. On a BIOS/MBR computer, once the 4 Primary partition limit has been reached, I don't know how it handles any more of those partitions, maybe these are simply upgraded. With UEFI/GPT, it doesn't matter, as the limit is 128 Primaries, of which out of all of the folks I know, you're the only one whom I see may in the future may need to add another drive. 
In your case, probably externally, hope that you have an eSATA port on the exterior somewhere. Which in most cases are as fast as USB 3.0, because of poor implementation of controllers (why we'll seldom, if ever, see USB 3.0 come close to it's standard). eSATA is a universal standard & maintains speed much better.
Or you can do away with your optical drive (if installed) & using a bay adapter for this purpose, add another drive of your choice. And then use a $5 (in USD) to obtain a USB to notebook optical drive cable, making it a portable one to use anywhere. If desired, can buy a case also, and you'll have a better portable then the common $25 models. 
About the 3+GiB externals, depending on whom built the MB, there's often an 'unlocking' tool that allows users w/out UEFI to use these larger drives, though being that the ones I've purchased were already UEFI (all three of my own & at least 20 for others), may be for those wanting to run an older OS. Or run in Legacy Mode (CSM).
Now back to the OP and the Topic, hopefully things are being sorted out. It's inadvisable to 'blow away' the OEM OS w/out a way to revert. Even if I were to purchase a computer (probably a notebook only) to run Linux on, if the seller can't provide a way to fresh install to out of the box condition, price drops. Like purchasing a car with worn brake pads or semi slick tires, and I check things before purchase. Price drops for items that needs repair, or in this case, missing full recovery solution, and I'll test before purchase, be it 3 months or 12 years old. 
Sure enough, one person tried to get over on me & failed, gave me a set of recovery media, and I said 'let's test it', sure enough the media was rejected. By the time I was through, a notebook listed for $200 in the local newspaper was mine for $60. I then called Dell and for the third consecutive time I needed this media, got someone in the US who was compassionate, gave the Service Code, was registered & then transferred to me, and was shipped a complete factory recovery disk set at no cost for Windows 7. 
Later on, about a year ago, I resold the notebook for $150, and that was after swapping both the RAM (8GB) into one of my own & replacing with a 4GB set, plus a swap of the 750GiB 7200 RPM HDD w/16MB cache (SATA-2) for a 200GiB, 5400 RPM, SATA-1 HDD with a 8MB cache, as well as using the 1st gen stock i5 CPU for upgrade in someone else's computer. In addition to a gift donation, I was told to keep dual core Pentium, which was also installed into the notebook.
Then cloned the OS to the smaller drive using Macrium Reflect, and set Recovery to Active & this forced the OS to reinstall (special thanks to pcpunk for this tip). 
I'm positive that the OP will find a way to install Windows 10 inside of a VM in the Linux OS, even if it requires joining the Windows Insider Program, as far as I know, hasn't been cut off, and get W10 at no charge (Microsoft Account required), if one has Outlook or Hotmail, a MS Account is established already. That would be sweet, to dual boot & have a virtual W10, which I had both on my XPS 8700, will be placed back into service, right now my priority is obtaining a 4K UHD monitor. 
http://www.howtogeek.com/224826/how-to-become-a-windows-insider-and-test-new-windows-10-features/
https://insider.windows.com/
Of course, either of the above options has to be established before creating a VM in the Linux OS. I still maintain that if one has the technical skills to get it working, use the vhdx option to mount the virtual disk of the complete OS (after being shrank to a manageable size & disk image created prior), to run in VirtualBox or VMware Workstation Player. One requires a vdx file, the other vhdx, and this option is in the Disk2vhd site I posted on the prior page. This takes some time to complete, so make sure of which is needed for the VM solution.
That is, if it doesn't install using Windows 10 install media (the Dell reinstall DVD may not work for this). The rules has changed so much in the last 3-4 years, it's difficult to keep up with them all, especially when not using the option. Actually, this PC I'm now on is registered for Windows 10, since I installed twice, may have to try & see how it goes. Nothing to lose & everything to gain, especially if it helps others who downgraded back to Windows 7 due to driver issues, which wouldn't be an issue in a VM. 
Cat