OK. I'm a bit surprised Windows 10 is on a GPT partition, normally that's used for Servers. However I browsed around online and there are a number of suggestions, some using third party software, probably not of much use, and others various command line options.
I searched on Bing using this entry
"gpt partition style windows 10 install" and there seems to be a number of suggestions, including some YouTube videos.
Is the hard drive a standard SATA drive / SSD drive or a motherboard mounted M2 drive. I take it a new install of Windows isn't much of an option if you have personal files you need on the drive.
This page may be of help if you scroll down the PART 2.
https://www.easeus.com/partition-mas...partition.html