Yes, I can set GPIOs flashing the mentioned official image to the board, however I need custom image as my I2S HW is not supported in any precompiled images.
The problem is that compiling the LEDE repo by default set GPIOs 14-17 to MDI_xxx (act as Ethernet ports) instead of GPIOs.
I can set those pins to GPIOs with devmem like stuffs at or after boot but that's far from elegant.
Regarding gpioctl, I did not download it directly, but installed by ./scripts/feeds install gpioctl-sysfs (from the top level directory of the installed SDK). However, it is available as part of libugpio. But it is just an interface to access /sys/class/gpio/*, it simply reads / writes those files.