How to modify RO permission to RW having root access

Hi,

My phone is Xperia Z1 Compact / Lollipop 5.1.1
I've tried to modify /etc/gps.conf using FasterFix app --> failed
I've tried to modify it manually by using Fx file manager (with root access allowed, of course) --> failed, read-only system file.
I did the same using adb mount -o remount,rw /system --> permission denied
I tried to modify permissions using CHMOD --> permission denied

When typing su in adb, root access is granted.
SuperSu is installed, busybox too.

What am I missing ?