Actually
gdisk did it perfectly, and quickly.
What I did:
Downloaded gdisk for windows and extracted to my boot disk
ran CMD as admin and navigated to gdisk folder
used disk management to verify that my disk to convert was Disk1
ran gdisk with the 1: argument ( with "1" being the disk number. "x:" is just the shorthand access)
entered "y" to confirm I wanted to do this
"r" to recovery transformations menu
"f" to load MBR and make GPT from ir
"m" to return to main menu
"w" to write to disk and exit
new disk connected dialogue popped up, but otherwise it's identical (except GPT)