Got this solved by myself.
Here is how I did,
Downloaded aRoot, modified the unroot.bat executed it and made a factory reset of the phone (which was not working previously). And GFE installed without issue.
For info here is the modified part of unroot I used (to replace what code is after the mount command :
(note that I added also rename of the droid.ogg to avoid the horrible noise when starting the phone, it remained silent even after factory reset)
ECHO Unrooting your device ...
adb.exe shell rm /system/bin/su
ECHO Removing Superuser app ...
adb.exe uninstall com.noshufou.android.su
adb.exe shell rm /system/app/Superuser.apk
adb.exe shell mv /system/media/droid.ogg /system/media/droid.bak
adb.exe shell rm /data/local/tmp/psneuter
ECHO Cleaning up onclickroot mess ...
adb.exe shell "find /system/xbin -type l | xargs rm"
adb.exe shell rm /system/xbin/busybox
ECHO Rebooting ...