This script (Modify Crosshairs Script) add only a vertical and horizontal guide at 50% of the layer and is NOT found in the menu Image > Guides > Crosshair Guides,
but instead in Layer > Guides > Crosshair Guides as specified inside the script
(script-fu-register "script-fu-crosshairs"
"<Image>/Layer/Guides/Crosshair Guides"
"Add Guides at 50% Horizontal and Vertical"
To add guides at the mouse pointer, use the Measurement Tool. (the one which looks like a compass in the tool box)
I quote from the link above
Ctrl key pressed and click on an end point creates a horizontal guide Alt key and click on an end point creates a vertical guide. Ctrl+Alt key combination and click on an end point creates a vertical and a horizontal guides.
There is a caveat depending your Desktop Env, for example the Alt alone does not work on my distro (Ubuntu-MATE), nor on my kids distro (MX Linux/Xfce), but doing a proper Ctrl → + → Alt in that order will put the vertical and the horizontal.
one of the GIMP’s Team member said it works fine on Debian with GNOME, if it’s your case you’re on the lucky side
Hope it helps