Thanks a lot to both of you,
I was trying to figure this out by myself and was stuck in my script-fu attempt since few days but your python script helped me to find out what was wrong on mine, thank you very much.
The python one works nicely, thanks a lot for this ( and i will have to check the gimpscripter link, might serve in the future too 
)
Here is my script-fu version :
;; Gmic_In_Main_Menu.scm
;; Simple script to integrate a shortcut calling G'mic-QT on the main menu of Gimp.
;; Seems like it needs to be clicked twice to be triggered tho.
;; Based on the example found here : https://github.com/dtschump/gmic/blob/master/src/gmic_in_script.scm
;; By Yalba @ pixls.us forum - December 2018.
(define (Gmic_In_Main_Menu image drawable)
;; Start undo group.
(gimp-image-undo-group-start image)
(let* (
(copy-layer (car (gimp-layer-copy drawable TRUE)))
)
(plug-in-gmic-qt 0 image drawable 1 2 "")
)
;; Flush display.
(gimp-displays-flush)
;; End undo group.
(gimp-image-undo-group-end image)
)
;;Registering
(script-fu-register "Gmic_In_Main_Menu"
_"<Image>/G'MIC-QT..."
"Shortcut to call G'MIC from Gimp main menu...
! Might need to be clicked twice to trigger it... ! "
"Yalba @ Pixls.us Forum"
"Yalba @ Pixls.us Forum"
"December 2018"
"*"
SF-IMAGE "Image" 0
SF-DRAWABLE "Drawable" 0
)
Seems like it does the job, this said placed like this directly on the main menu it needs to be clicked twice to be triggered, if placed on a submenu like in your python script it works directly, not sure why, same thing happens if i modify the python script :
"<Image>/G'MIC/G'MIC QT...",
by
"<Image>/G'MIC QT...",
Modified like this it will need 2 clicks to launch Gmic. 
Anyways it’s good enough like this for both the python or script-fu, result :
Now that i figured out this i might create in the future a few more script-fu scripts executing directly some of the gmic commands i use the most 
Script to put in AppData\Roaming\GIMP\2.10\scripts\ ( for windows that is )
Gmic_In_Main_Menu.scm (1.2 KB)
Thank you very much for the help