I finally found that I don't need SSDT-GPIO, and found another way to enable f11-f12 brightness keys.
This is based on SSDT-BCKM from tctien342 on GitHub ,so credits to tctien342;
he has commented:
// make BRT6 to be called on Darwin
// call chain: _Q66 -> NEVT -> SMIE -> SMEE -> EV5 -> BRT6
// in SMEE, EV5 is called only when OSID >= 0x20, and OSID:
// if ACOS == 0: init ACOS based on OS version
// return ACOS
// hence set ACOS >= 0x20 can do the trick, and this trick affects less methods than _OSI renaming patch
I am not an expert and deleted some lines I felt I did not need, but this works on the 5570 and 5770 Inspiron; maybe you want to take a look as this could work on other Dell Laptops as well.
My EFI setup using OC 6.7. and everything appears to work fine:
