GPT is not a file system, so it's somewhat unclear what you did.
What does Windows' disk management say about the disk? And what does GPARTED (run it from any Linux disc or stick - free to make) say?
Surely you have a backup of the data on that partition that you didn't want to lose?
I was installing Windows 10 on another computer and I could not install due to 1 of the drives being MBR not GTP. I have found online how to convert the partition to GTP and I have done it via windows installer (shift + F10). After I cleaned partition 0 (c) and change file system to GTP I can't see me other D drive.
Anyone knows how to fix this? I appreciate any help. My HD now instead of 1TB is 300 GB

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