OK so sorry about the noob and dump questions
I solved it by chaning to this:
{{(itemsInGroup('gLiving')).length + (itemsInGroup('gKitchen')).length}}
A simple + solved my pretty dump questions… and I wasting my whole day trying to make this right 
But still, I am wondering what cause the wrong counts of using just {{(itemsInGroup('gLight')).length}}
I thought gLight is the root group, it should counts the total items inside gLiving and gKitchen, it should returns value 5(lights, or items I should call), not value 2(two sub-groups).. why?