I tried installing kmod-gpio-dev kmod-gpio-dev_4.4.50-1_mipsel_24kc.ipk and the file was downloaded and installed with no problems or errors except that it didn't contain any kernel module at all. This was confirmed by extracting the .ipk file and there were no /lib/modules/ inside it. I guess this is not the only package with this problem.
Another thing: why some modules such as: kmod-usb-ohci-pci and kmod-usb2-pci are available for some targets by default (ar7) and not for other targets (ramips)? is there any good reason for this or this is just the way things are?
I can't upgrade to the latest LEDE because of flash and memory size limitations. Anyway upgrading will not solve the problem: the modules I need will still be missing there.
I tried custom building the needed modules but that didn't work. Lots of error messages as I needed to build a specific version and not the latest version.