In that case, GRUB really just chainloads the windows bootloader and windows doesn't come with a meaningful amount of GNU packages (windows comes with enough libraries to provide a "working" computer).
Meanwhile, the kernel, Linux doesn't even provide a bootloader (although there are separate bootloader projects that have "Linux" in the project name), nor any usable libraries - you need to add a base of GNU, or an inferior GNU clone like BusyBox to get a working computer.
It seems that windows may in fact come with some GNU packages under a version of the Lesser GPLs, as well if you go to; Settings > System > About > Read the Microsoft Software License Terms or https://learn.microsoft.com/en-us/windows/iot/iot-enterprise/eula/license_en-us_english_united_states.pdf you'll see;
"you may not (and you may not permit any other person or entity to ... reverse engineer, decompile, or disassemble the software, or attempt to do so, except and only to the extent that the foregoing restriction is ... required to debug changes to any libraries licensed under the GNU Lesser General Public License that are included with and linked to by the software".
Also, certain versions of windows were developed with GNU packages like GNU make for the buildsystem (until that was replaced with microsoft's internal garbage buildsystems that makes it a nightmare to add a drop-down to settings for example).
If you want to really stretch it, you could say that windows 10 is GNU based, but that'll disappoint people if after hearing that, they decide to look for the GNU/Freedom in windows 10 and find none.