Come to think of it, I may have f'ed it up already. When I opened Disc Manager it notified me of the new drive and asked me to choose between MBR and GPT where i selected GPT as it was the default choice. Does this matter in this case and if so is there a way to change it to MBR?
Yep that would do it.
You can give TestDisk a spin and try to recover the original.
In any case, whatever you do, it shouldn't be done on the HDD that was fked up. You should make an image of the disk and then try the recovery on that image... Two potential problems with that approach:
1) HDDs are kinda slow.
2) Free space. 1TB messed up drive means you need 1TB free space on your PC for the image plus another 1TB free space for recovered files.
If the data isn't important I would just yolo it directly on that messed up HDD and if it succeeds - great, if it doesn't I'd just format it and re-download stuff.