Some keyboard related software like Karabiner for macOS and the QMK firmware uses the names left_gui, right_gui, KC_RGUI, KC_LGUI for the Command key. And I found that also corresponds to the Windows key if using Windows OS.
Where does the name GUI come from historically?
The name GUI comes from the USB HID (Human Interface Device) specification.
The scancodes/keycodes for the keys labeled Windows/Meta/Command are named GUI in the USB HID specification. For example the section 8.3 Report Format for Array Items of the USB Device Class Definition for Human Interface Devices (HID) 1.1 spec says :
For example, key such as CTRL, SHIFT, ALT, and GUI keys make up the 8 bit modifier bytes in a standard keyboard report.