If you don't shy away from piracy, or have money to throw away, you'll find GetDataBack for NTFS rather useful. At this point, you've killed your MBR and your PBR, as well as your MFT, so you'll only be able to find your file via pattern matching. Meaning, the software you should use has to recognize a file's header to discern whether it's a .jpg, an .mp3 or an .avi, for example. And even then, it will be hard to recover badly fragmented files, if not impossible. Why were you doing all that in diskpart? Actually, why the hell would you convert a disk containing important files to GPT? Did no one tell you this is a destructive procedure?
As for using GetDataBack, be sure to set the options "Sustained file system damage", then use "Excessive search", "Recover lost files", "Recover deleted files", and un-check "Use valid MFT entries only".